Designing Kitchens and Bathrooms - Framing a drop-in tub enclosure for tile

I am building out an enclosure for an air-spa tub. The framing instructions show sizing but not materials.

Questions:

1. For the framing should I use pressure treated for anything that contacts the floor? Presumably all 2x4s. Do you shoot these down with a stud gun or is there some better way to attach?

2. Once I build the wood frame I have a small area around the top and one side to tile. For the horizontal top surface and the one side I plan to tile do I just cover these with backer-board over the studs?

3. I've read about putting a bed of thin-set under the tub. Is this necessary? Advisable? Is some type of strapping necessary to secure the tub to the framing?

Can you tell I'm a novice....??

Thanks!

Do you have the tub in place, now? Usually these tubs have a plywood/osb base, which you can secure to the floor, so a layer of thinset won't be necessary. The last one I enclosed, I measured out from the edge of the tub an additional 2 3/4", and used 2x6's for framing. Basically make a base of pt lumber, a top plate and "studs" to build the frame to within 1" of the underside of the tub. Install 1/2" Durock on all surfaces (sides and top. Remember the tub is free standing and will only "lean" on the horizontal tile lip. Now, you can tile the sides and slip tile under the tub lip and onto the horizontal surface. Check out the photo at http://i76.photobucket.com/albums/j3/larrychandler/DSCF0488-2.jpg and see if this is similar to what you are attempting.