Castle Creations motor and ESC combo recommendations for the HPI Baja 5b:

Cobra 5 / Mamba XLX2 w/2028-800kv - Stock Gearing (14/57) on 8s = ~48MPH
Cobra 5 / Mamba XLX2 w/2028-1100kv - Stock Gearing (14/57) on 8s = ~66MPH

Note: All speed estimates are with stock tire diameter, high quality batteries and appropriately sized battery connectors.
Always monitor ESC, motor and battery temperatures on new setups or when changing gearing or batteries.