5955 is SUPER for steering.
I have used a 645 for throttle/brakes in my Revo, and in a truggy on short notice. It is better to get one a little faster or stronger, or both for throttle, but it will do until you can afford a better one.
If oyu are going to transform a truggy into a buggy, that is EXPENSIVE, better off selling the truggy and buying a buggy if that is the case.
Most guys like truggy better, it is very fast, very easy to drive, and is much more stable. I prefer buggy myself. You can find used buggies for a fairly good price on here, as well as other forums, and if you son't like it, you can usually get close to what you paid for back out of it.