Well, in general Associated and HPI are in almost every hobby shop. I'm fortunate enough to live in an area where I can get parts for Mugen, Kyosho, HPI, Associated, Losi, Ofna and even Yokomo and Serpent. This is just a guess but here is an estimated population break down of cars in my area
Alot of the sedan guys here are also 1/8 scalers and they run MRX3s and it's probably why they also run MTX3s...brand loyalty. V1R and NTC3s are probably really close so I put them both at #2. In your area I guess you'd have to find out what most people are running but there is tons of online support for all these cars right here. For parts, you'd have to check with your lhs to see what they have. I purposely got the HPI R40 because 1) not too many people have them. 2) I'm not good enough to where a car would make the difference between me winning and losing. 3) I'm a hop-up freak and HPIs are notorious for 3rd party support. 4) I can get HPI parts easily.
Engines, I would go with a Nova (NS12, NSR12) or Nova based engine like RB (V12) or Top TS12.