I went with the RCgear shop 3900 kv for$29 at Tower and the Raider BLX ESC. It is plenty fast ..... and thats on 8.4 nimh.
Mine is the original mega not the XL so yours may be a tad heavier but that's what I would recommend for a "budget" upgrade to brushless! (That ESC may only handle 2s). Some of the other guys may be able to chime in on that.