Doors and Windows - Is this a doable beginner project?

We have recently purchased a 10 year old double wide, and we'd like to upgrade the windows and doors. We have basic power tools, enthusiasm, and very little experience! There are three doors and a dozen windows. I'm curious which if any of these projects we should tackle or whether a professional would be a better bet.
Door A--looks something like this (http://attachments.techguy.org/attachments/88515d1159541112/door-001.jpg), but that trim around the glass is all yellowed and ugly looking, and it needs at least a hardware/lock upgrade. I think we can handle that, but how about the yellow nastiness? Can that be painted or fixed somehow, or is indicative of a cheap door in need of replacement?
Door B--Same as door A, but we'd actually like French doors in it's place--that'd involved enlarging the opening significantly and scares me a bit--we've done interior/non-structural/new-construction framing, but nothing remodeling/exterior/important! Should we hire this project out, or is it something we can work up to?
Door C--a nasty trailer door--apparently the 'escape hatch' door is not required to be a real door by HUD, so they put something even I could kick open here...it's not a standard looking size, so that'd involve some enlarging as well, although on a much smaller scale.
(Looks something like this: http://www.hillvalleymobilehomeservice.com/nss-folder/pictures/Door%204.jpg )
As for the windows, this is the closest picture I could find online, and ours aren't quite as nice as this...but just so you can see, they're cheapy vinyl-esque windows, but all basic rectangles. http://www.detrayscustomhousingllc.com/customize/2%20Trap%20windows.jpg

Our ultimate goal is to improve the aesthetics, energy efficiency, and security. We have a year to fix it up before we move in (so we don't have to live in it while it's under construction) We plan to live in this place for 15 years while we pay off the land it's situated on before building our dream home, so we don't want to sink a fortune in it, but we want to be comfortable living here, as our children will do most of their growing up in this house. We are on a shoe string budget, so the more of our money that can go to materials as opposed to labor, the better, but if it's in our best interest, we'd rather delay a project and hire a professional than do a crappy diy job ;)
Oh, and any recommendations on brands/quality of products we should use, given the 15 year life-span of this home? (it'll be demolished/sold for scrap/given away when we're done with it)

You will find in retrofitting a double wide, regular framing just won't work, since the wall thickness is different. As far as door A, you can paint the trim, even though it is plastic. It is yellowing due to age and sunlight which is normal. Retrofitting for a french type door will probably not be something you can do as diy'ers, since the framing of the home will need special support, and you don't have the space for a normal header to span across the door opening. Door C will require removal, reframing, installation and retrimming, but the door frame will need to be cut to fit the opening due to the wall thickness and probably the size of the door itself. Windows can be replaced with similar windows from a mobile home supplier, or special ordered with proper jambs from a window supplier (here again, taking into consideration the wall thickness). Are the present windows single pane or double? If double, are they fogging? If they aren't broken or fogging, you may want to put this part off for a while to save a little initially and concentrate on the doors.

Most mobile home exterior walls are close to the thickness of conventional home. Depending on the siding used they are 1/4" - 1/2" narrower, so installing new windows is doable. Most MHs use a double storm window - 1 storm window on the exterior and another [turned around] on the interior.

As Larry noted the exterior wall height is usally only 7' which leaves no room for a header with the use of a 6' 8" door. MH supply centers sell MH doors which are only 76" in height, including french door units.

Thank you very much! I do know it has 'normal' framing (2x4 walls) but I hadn't considered the exterior wall height--they are 7 foot exterior walls (cathedral ceilings, so it doesn't feel claustrophobic). The siding is cement-fiber hardipanel, like this: http://www.jameshardieeu.com/pages.php?pages=products&subpage=hplank
Ok, so if I can find a suitable french door, is that something I call a general contractor for an estimate, a handyman, or someone who specifically does windows/doors? Is there a best time of year for getting someone to take such a small job? (We're near Dallas, for seasonal considerations...winter is Dec-Feb)
The windows I THINK are double storm as marksr described...none of them are broken or fogged, they just seemed a bit flimsy. I didn't inspect them closely, we haven't taken possession yet, so we've only had one walk through as yet. I'm not in a rush to replace them, but we'd like to do as much of the dirty/disruptive construction before we move in. We plan on painting, putting up trim and installing new flooring and other basic aesthetic stuff ourself, we've done that type of work before. The doors and windows are new territory for us. Good to hear I can paint that yellowed trim, that'll certainly save some dough. Is that trim around the window removable, or do I just tape carefully? Do you generally roll doors? Any recommendation on the type of paint?
Alrighty, I do believe I've pestered y'all enough for one day! Thank you SO much for all your helpful advice!

If I understand what you are asking - the metal around the windows is part of the "storm window" While it could be removed it would be easier to leave it in place.

More than likely both the doors and walls will need a solvent based primer. The drywall is a little different than normal drywall - it is only 1/4" thick and instead of having a paper face [that you finish and paint] it has a vinyl facing - usually with a print that mimics wallpaper. Using a solvent based [oil or shellac] primer will insure that the paint will adhere. You will want to open the windows and air it out during and after the priming.

Doors can be rolled or brushed. Often we'll roll the paint on the doors and gently "tip" the paint off with a brush to remove the roller stipple. If you plan to only roll the doors you will want to use a 1/4" nap roller cover - leaves less roller stipple.

As always the best paints [advice too] are found at a real paint store. While the paint might be cheaper [per gal] at a big box, not a lot of money is saved when using quality paints makes the job easier - covers and applies better :)

I just realized you might mean the "yellowed" trim on the door - it is best left inplace.

The exterior walls should all be 2x4s but the common middle interior wall is basically a 2x6 cut in half and then joined back together. The other interior studs can be anything from a 2x4, 1x4 or 2x2.

I've been reading through the older posts and saw someone suggest taking exterior doors to an auto-painting place to have them sprayed? Any suggestions there? I'm guessing this is done because it gives the smoothest finish? Is it also more durable, or just an aesthetic thing? If it'll be more durable, I'll do that, I don't relish the thought of repainting the door very often!

I'm the one that has suggested that from time to time. We do it on some of the primed prehung doors that need to be painted. The coating they put on is both durable and a quality finish- pretty much just like what your car has on it. None of the doors we have had painted this way have ever needed repainting. Only drawback is that it's fairly expensive.

I've sprayed some doors myself using a waterborne industrial high gloss paint, and have been very pleased with the results. So it all depends on which route you decide to go. More than one way to skin a cat, they say... although I don't know what kind of people skin cats? :(