Solidus, listen to docb. He has direct experience and knowledge of DJ turntables.
I had a look at the SL-1200 manual. It confirms what docb says, that it's direct drive. It uses a three-phase motor with three drive coils and a ring magnet. The relevant part of the schematic is the bottom right corner. IC101, type AN6675, drives three coils, and there are several other coils for feedback.
The SL-1200 specifications say that the drive is "quartz direct drive" and that it uses a "direct drive brushless DC motor". You can see it on the PCB layout. It's quite large, and I agree with docb that you would need a good diameter to get good torque at low speed.
So belt drive is not feasible, idler drive might be, but direct drive is the way to go. It has all the advantages - high torque, and accurate and easily variable speed. It's not really practical to build it yourself. Perhaps you should consider buying a cheap turntable and making improvements? I don't know.
docb, what are your general thoughts on what Solidus wants to do?