I scratch built my turntable, but it's an Armstrong model (no motor). I didn't use any plans, it consists of some Atlas bridge girders mounted to a block of wood. I originally mounted an old electric hand mixer under the layout and had planned to turn the motor with a hand crank extending through the layout fascia. The bridge is mounted on one of the beater shafts. Because it's very close to the edge of the layout, I eliminated the mechanism and opted for "big finger" operation. Power for one bridge rail is provided by wipers under the layout contacting the beater shaft, the other is powered through the pit ring rail via wipers on the table wheels. Indexing is by eye.
I'm not sure if the speed of a prototype turntable can be regulated, but I have seen one operate so quickly that the diesel that was being turned actually rocked when the table stopped. It looked very unprototypical.
A very good, but pricey, rtr turntable is offered by Fleischmann, complete with indexing. For kits, Diamond Scale, also expensive, will build into a nice model if you don't mind correcting some minor design problems. Their indexing system is also pretty good, with the bonus that you can set it up at whatever intervals you require.
Wayne