The easiest way to convert a plastic model to RC is to use a simple 2 channel radio. Use one servo for rudder, and disassemble the other for a combo speed control/powerplant.

An excellent subject for this treatment is the Lindberg Diesel Tug. At about 13" LOA, the tug has enough bouyancy to make a fine RC model. (Lindberg apparently noticed and even released an RC version of the kit.) The Lindberg North Atlantic trawler and the Lindberg Fishing boat are also good subjects for mini-RC conversions. The Lindberg LCVP is another good subject.

If you are starting with a motorized model kit, the first thing to do is to throw away the motor that comes with the kit. These are toy -grade motors, and are very cheap. They are very noisy- they will generate lots of radio interference.

The Revell (ex- Matchbox) kit of the FLower class corvette makes a terrific RC model as well- I used a 6 volt Pittman motor and direct drive.

Personally, I find the hardest part of RC conversions to be obtaining a decent small diameter prop. When using a servo for a motor, a lightweight prop is better- plastic is better than metal. Problem is, a reliable source of decent props smaller than 1 inch diameter is hard to find. One source is inexpensive battery operated toys.... (Loyalhanna Dockyard is a good source for metal props.)