Doors and Windows - framing for hanging a door

I bought a home recently, and the past owners partially finished the attic. In finishing it, I want to instal a pre-hung door. The current opening is too small for any door sizes, so I bought the next biggest size. There are studs on either side of the opening that will have to come off when I cut the sheetrock to enlarge the opening. As this was new construction, they didn't have any studs near the door other than these. The back of the current wall is open so that I can install new studs for the door and even the wall itself without having to take down any walls. After opening the wall, since there wouldn't be any studs to use for the support of the frame, should I use 2x4 or larger studs, and should I use more than 1 on each side? As there is also no current stud to use as the header, what should I do about this? Should I run a stud or 2 from the header to the rafters to steady the header and the door from sway? Any other suggestions?

I assume that the wall is already framed with 2X4 studs.
Double the studs on both sides of the door frame, and build a header above it of two 2X10s with a piece of 1/2" plywood sandwiched between them as a spacer to make up the 3.5".
The framing should be tied in from the sides and up to the rafters for stability (unless you're adding a ceiling).
lefty or Jack the Contractor (pros, I'm not one) can give you more detailed advice, but that should get you started in the right direction.
Good luck!

What, if anything, is this wall currently supporting? THAT will determine the size of header you will need. If this wall runs perpendicular to the ceiling joists or rafters above it, it is probably a bearing wall, meaning it is supporting your roof. If that is the case, hire a contractor to frame in the door opening for you. The few hundred dollars he will charge is a whole lot less than the tens of thousands of dollars of damage you can do by not doing it properly. (Why do I feel like I'm repeating myself?)