I tried to race with a hv 4.5 last year. That was definately not a good idea.
This season I have been running the HV Pro 6.5 with no ill effects. The track is big but technical, there is only 1 long straight everything else is not very high speed.
The buggy can definately keep up with with the nitros, just make sure you are following novak's gearing suggestions.
I just picked up a CC BEC to try to eliminate the issue of one of my dual packs ends the run w/ 0.1V lower Voltage (onboard BEC only draws from the "batt 2" plug, but can only handle 2s lipo).
I haven't had any problems with the Novak onboard BEC, but I am hoping running an external BEC with extend the ESC and battery life (ESC=lower temp, batteries- more balanced).

For a buggy I think the Novak system is a good choice. It doesn't give you so much power that you can't handle the car. If you are a pro on the track you could probaby handle a MMM or Tekin, but it would be overpowered and hard to handle for a beginner.