I have whisper warm and we installed a timer switch for the fan portion so we dont have to remember to turn the fan off when we are leaving the bathroom. Works great. I was told it cant go inside the shower though so ours is right outside where you step out and the heater is great. I too bought one bigger than the space called for and have no regrets. I read that some complained that the heater didnt warm up enough and that made me get the larger size. The heater is great! Remember though that it takes a whole circuit breaker of its own-20 amp if I recall correctly so make sure you have enough space on your electric box.

It's possible to get replacement bare motors sometimes. Broan are good about having replacement bases -fan/motor, that fit into the existing box. They have a plug/socket, so you don't have to do any wiring. That being said, I just replaced one, and it turned into quite a big job because the replacement base was more than a whole new unit. I put in a new one, which meant a bit of carpentry etc because the boxes were different sizes. I had to cut the nails/screws holding the old one in, and insert the new box from below, which required a bigger hole, and careful saw cuts in addition to allow for the nailing tabs. The fan cover covers the repairs from it, in the ceiling. The insulation around the duct had no vapour/air barrier so I used a mylar bubblewrap product and foil tape for it. Since the house was old enough to not have had an vapour bag around it, I built one with the mylar stuff and sprayfoamed it in. Lastly I decided the fan should have its own switch, so I could use one of those timer/humidity sensing ones. It turned out well and I'm glad I did it, it works better and is much quieter. But in most cases, just replace the fan motor and base.

To answer your question, we'd need to know where the individual cables that connect to the existing lights run. Do you know that? There are more than one possibility for wiring of this circuit. I'd recommend buying a book, a trip to the library as well or some internet surfing as well. Do you have space in your service panel or a sub panel for a new circuit? Since you are down to the studs, this is a good time to do it. You should have a dedicated circuit for the convenience outlet(s). I believe that the lights are on a separate circuit from that, but I am not sure.

Be aware that plumbing requires from 1/8" to 1/4" slope depending. You can hide it in a coffered ceiling, but that would require a check on the total slope and great coordination with the design. Estimating from room listings you might be traveling 35'+ to the back, so that could be 4"-5" up to 9"-10" of slope, in addition to the size of pipe being 3" or 4". You might be better served by going down through 1st floor walls and running across the basement ceiling. However, the 6'-4" ceiling height listed is suffocating, if your main to street is low enough you might be able to trench the basement floor and run it below the slab, or install an ejector pit.