A battery operated screw driver makes free 100 rpm, 3 volt dc powerful, reversable gear motor. The cheap ones are $15 new and in a few years, the sorry rechargable NiCd batteries are shot and it is not worth the cost to replace them. I have a B&D one myself and have picked up two Skil screw drives for 25 cents each at garage sales (OK almost free).
Power them with a $5 HO dc transformer. This things make the perfect turntable drive motor with 100 rpm, lots of power and Reverability is what you need for turntable drives.
An extra: The cheap 12 vdc electric drills make great drives for model boats and are variable speed. I picked up a Bosch drill, 14v with two batteries and charger for $3. The batteries were weak and only lasted a year or so. I bought a 14v battery and charger for $10 at Harbor Freight and modified it to fit. The original batteries for these higher quality drills are $40 a set so lots of folks tra***hem.
Charlie