Rich, the mtx conversion is 200mm all other ep cars are 190mm.
Go for the t4 it will suit you, and it's new for 2014. IMO if you go with anything else, awesomatix or yokomo. The awesomatix is almost bulletproof as far as durability and so is the T4, the yoke isn't but it's really my favorite car just have to order parts no one local carries anything for it. If you find yourself at horsham, xray and awesomatix parts will pretty much be there so go with one of those.
Carpet racing is fun and really improves your driving, after a season of carpet my nitro driving spoke for itself, Rich you are my witness. See you at horsham