replacing the bathtub and changing the supply lines

I am replacing my tub and also changing the supply from the existing three knobs (hot, cold and shower head) to a single arm control for on, hot/cold adjust and a 'pull up' tub filler to change water from tub to shower.

I am also changing the drain from a lever on the tub to a pop up drain with a blank installed where the lever used to be. I am tearing out everything to the studs so access should not be a problem.

I want to know the best way to install/change the drain piping (and at what stage of installation do I do that). I bought the drain 'kit' which comes with PVC elbow,12" tailpiece, coupling and everything 'they' say I need to do it but.......??? I also need to know the best (and proper) way to change the feed lines over from handles to the diverter valve.

Should I tear it all out and start running copper from the floor or can I adapt what I have?

Lastly I am putting in a fiberglass 3 piece enclosure above the tub. I know back wall first but is there a land mine waiting for me to cut the hole for the shower head and the single handle control?

I am very handy but I am apprehensive because of it being plumbing and the unknown. Plumbers want 500.00 to do just this with me doing all the tear out and the install of the tub and walls. I can not afford that on top of the material costs I have already spent. HELP
Bob Smith

All Tips are welcome.