Designing Kitchens and Bathrooms - basic shower install questions

I'm planning on redoing a shower room by replacing an old leaking corner stall with a one piece 48x36 inch shower stall. My questions are, I have some water damage under the shower base, so when I replace part of the floor, should I use 3/4 treated plywood? And do I put some kind of moisture barrier over it before installing the shower on top of it? any other suggestions or tips would be helpful. Thanks.

No to PT wood in the house, expands/contracts too much, yes to vapor barrier. But how will you get that one-piece unit into the house. let alone into the room???? Those things usually have the house built around them.

OMG!! I can't believe I didn't think about that! I was just about to leave and go buy my shower when I decided to check my thread before I left. You just saved me alot of headache. I would've felt real stupid when I got home and tried to fit that shower though the door! So I ended up buying a three piece unit same size. I just wanted to not have to worry about sealing seams. Thats okay though.
So for this vapor barrier...Can I use 4mil plastic? Just laying it under the base of the shower, and tacking it down with stapels? How would you go about it?

No intentions of jumping on Bills answers, but the 4 mil can be tacked down without problem. Now, don't forget to put a group of globs of thinset equally spaced to bed the base in. This will give good support and it won't feel so springy. This can be placed on top of your 4 mil.

Well, this shower is only temp. I want to build a full size bathroom with a shower and whirlpool tub in a couple of years. The current shower room will turn into linen and laundry closets. So I want to be able to remove the new shower and use it in the new bath. Is it possibe to do that with the thinset seating the base of the shower? Without damage to the shower??

Do you think it'll be okay without the thinset??