If you look round at what people are currently using in the UK nationals, the most popular engines are between Novarossi and Bullitt.
My recomendation would be a P5 XLT, less power than the Plus 4, and if this is your first foray into nitro would be a better choice. People are very quick to go and buy the fastest engine, rather than getting the car setup properly.
I would use the Novarossi 9901 pipe, with either the 021 or 029 manifold, depending on how much space there is.
http://www.jemodels.com/nitro-models...facturer_id=54
http://www.jemodels.com/nitro-models...facturer_id=54