1. I personally don't think it is required but for peace of mind it would be nice. It isn't too difficult

2. see below ...
A. You will need 2 power/ground wire (4 awg) from battery to amp. You will also want an inline fuse for one of those lines
B. You will need 12 awg speaker wire to run from your amp to your sub. Depending on the location of your current equipment, it may be easiest to just rewire all the in boat speakers as well. You could buy a large roll to do it all
C. You will need RCA's to stretch from head unit to amps. Also a remote wire to span the same distance.

3. Don't know but I have a 10" Kenwood Excelon I'd consider selling for the right price

Someone with more knowledge can chime in on more specifics. Some info that might help them ...
1. Where is your head unit?
2. Where do you plan to put the amp?
3. Where is your battery?
4. Where are you wanting to put the sub?