PSIturbo- That's one of the things with onroad - to find a nice spot to drive them. There's always some issue that prevents a car to be driven on perfect grounds. As for the PRP, it's still a good car and in capable hands can run with the rest of cars out there. For an engine, check out the OS motor as it's inexpensive.
http://www.osengines.com/engines/osmg2072.html
The Mega engine from Serpent is decent too. They claims the 06' versions feature a stronger crankshaft and conrod so it won't break as the older ones did. For more power it's a good motor, otherwise being on a budget I'd still get the OS and use the coupons off tower to reduce the pricing a bit. Overall an 8th onroad car is beefed up compared to a nitro touring car and can take abuse a bit more. However being bigger and stronger they also hit harder. They don't use foam on the bumper otherwise the lola bodies wouldn't fit. Speaking of the lola body that is the best body to run with period for these cars. Try anything else and they'd be a handful at any given speed. They are crucial in keeping the car planted. To make use of the downforce properly they have what's called a floating rear body mount. See the arms connected to the long body mount?
The force from the air flowing over the body presses down on the mount which sends the force to the hubs. Without this, you are simply pressing down the rear of the car offering less suspension action and can result in major understeer. I tried driving my nitro sedan with two different bodies to prove this: the Protoform Dodge Stratus 3.1 and the Protoform 200 mm Lola T530 (for nitro sedan). The Stratus body allowed my car to make use of the chassis setup better and basically felt more freely around the track which is good incase of slight errors. The Lola kept the car well planted and sometimes even too planted so some speed was scrubbed off. Anyway, back to 8th for Serpent there is only the Serpent Vector, Veteq, 950, 960 and 960R. As far as the RG goes it indeed is reliable and will get you use to it. Being on a budget is indeed annoying, but most importantly get things you need first then you can worry about RC and the other stuff later.