fiegos are cheap but they run hot (very hot I guess)
-Go with the MMM controller (castle has taken preorderes so I have to assume they will really release it when they say they will (end of may?) But I think the MMM motor is still in waiting. I don't follow castle, so I could be wrong.
-For racing 4s is sufficient, you could run 5s, but it would only be giving you more top end speed (which isn't what you really need on a track in my opinion).
-Most people go 5000mah to 6000mah, some go high (8000mah or more) to make absolutely sure they will have enough juice for anything. But a larger pack will add weight and slow you down.
-find an inexpensive roller, BUT it would be in your best interest to make sure a local hobby store sells spare parts for that brand/model.