here's a big pile of questions: need setup advice

[jnev]

1) 6s should be plenty. Its perfect for bashing.
2) It all depends on how you drive, but you can get about 40 - 45 minutes probably.
3) A 1527 will be way to powerful... you won't even be able to go full throttle, you'll just be wheeling all the time or break parts in the diffs/tranny. The 1521 and 1515 motors you mentioned has too high a KV in my opinion for that voltage. If you want speed but still be reliable, the 1515/2d would work well, or the 1521/1.5d. And they would both give you incredible speed.
4) The MMM will be perfect for this application.
5) Use this calculator to find the gearing you want: http://www.scriptasylum.com/rc_speed/_top_speed.html
6) Mike can sometimes special order them, but you can also buy them at maxamps.com or I am pretty sure amainhobbies.com also sells them.

Edit: He beat me to it.