You can run 2.2 buggy or truck tires / wheels - or traxxas 2.8 / proline 30's which I run mostly.
2 cell will be fine on any motor faster than 3500
Mod 1 - yes you need mod1 pinions and bore reducer - see below
http://www.rc-monster.com/proddetail...1pinion&cat=40
http://www.rc-monster.com/proddetail...reducer&cat=40
CRT.5 is a fairly chunky car - metal chassis - full tripple diff - the mambas are really suitable for 2wd statdium trucks and onroad cars.