I have no experience with the 4-tec, so I can't help there, but I know the Nitro RS4 2 and the RS4 3 quite well, as well as the NTC3. The best bang-for-your-buck (or pound, maybe) deal is the RS4 3 Type SS Kit. With a few small hop-ups (mainly a pipe) its a great kit for racing, at a price that won't kill you.

The Ofna LD3, in my opinion, is a sad thing. It may perform well, but its a total hack job on the Associated NTC3. To an untrained observer it might be hard to tell the difference. I've got little respect for the car, they couldn't even come up with their own design. Speaks volumes about the merit of the NTC3 though.

As far as 50+ mph on the track, thats a rare speed. The only case I can think of is running on a 1:5 scale track (like KZ Speedway), but on a purpose-built 1:10 scalre track, I would be surprised to see cars reaching 50, and even if so, it wouldn't be for very long.