First off, you want the 1512 cans. Anything longer and you would be wasting power and adding weight to your buggy. Anything smaller wouldn't be quite big enough. The 1512 is perfect for a buggy from what I have read.
Try the top speed calculator:
http://www.scriptasylum.com/rc_speed/_top_speed.html
With a 4S setup you'd want between 2500 and and 3300kv. For a 5s setup you'd want between 1800 and 2500kv. For 6s you'd want between 1500 and 1800kv.
-Jeff