simples, more KV = faster motor
however, in these small cars the gearing can make a huge difference, so anything above a 5900kv is probably way too quick!
also, if you go for a larger can motor, with a 1/10th size spindle, then a 3900kv will be super fast and more than enough.
best bets are to go with the ezrun/hobbywing sensorless 30/60amp esc, great value for money and most of the top micro drivers run them.
motor wise, 28mm diameter is pretty much as big as a motor the mini8 can take, whilst still being legal. although the stock motor is pretty good@ 4500kv.
best bet is to get on the micro forum
http://www.uk-microrc.co.uk/forum/ and scan through the threads there.