1. Yes it is important for obvious reasons...but if you don't bald beyond NW4 you can get away without using it..
2. Better to start before, some docs recommend using propecia for 12 months then get HT, which is great advise, and shows the doc has your best interests at heart as opposed to his bank balance.
3. Brand is unimportant. Recomended dose is 1mg, you can take less, but beware many have temprary side effects that will go away, this is pretty normal, long term effects are rare, but yes there are horror stories online, but isn't there for practically everything.
4. I don't know much about Acell.
5. don't think it matters, best not to focus too much on the tools, more the technique,skill, results of the doctor, although i'm not a fan of neograts or motorised FUE.
6. No idea
Hope this helps