That XL-5 should handle down to 12T motors, but I'd probably stay in the 14T-15T range to keep it from getting too hot. If you're on a tight budget, I've had liked the Reedy Flash motors; they're decent machine wound doubles for around $35.
If you can afford it go for one of the newer low-maintenance motors with the aluminum fins and round brushes. Something like a Checkpoint would work well or Fantom, Trinity Cobalt etc.
GV2 Pro, XXX4G+, Blitz SCT, CRC GenXL