Medusa 60mm motors have been succesfully run in a MT. They are more than adequate for a buggy. Gear for no more than 40 and you should be perfectly fine. Even if a Nitro can achieve more than 40 you'll still smoke'em! Brushless will pull so hard from every corner the nitros don't stand a chance. IMO a 70mm is overkill for a buggy and an 80mm would just be a waste!
The 60mm will even run truggies. Jammin' X1-CRT with 36-60-1600 geared 14/46 (buggy diffs) and 5s 5000mah Lipo. The motor doesn't exceed 160 deg. and tops out near 45mph with excellent performance.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz