Okay, it took me a second to understand what you were talking about, but its 2:35 Am here and I've been up since 7 yesterday morning. Don't ask me why, its been a long day.
Here is my answer to you. I do not have a pencil or paper handy, nor do I remember the formulas from school, but I think you can find out how much torque is required to rotate the platter by assuming the weight is all along the axis of rotation. Then you can calculate the torque supplied through the gear ratio you have selected. Pick a motor with an applied torque value just larger than the required value, leaving room for error, so say ~15% - 20% above the required value.
As my father has spent the last 20 years working with gears, and I have grown up my entire life around them, I can tell you this. Do NOT use a bevel gear. Use Helix gears, they are smoother, and loads more efficient. Also, you have the beautiful feature of being able to just slam the motor to a stop and not break the teeth off. They will be amazingly quiet, and are really the best bet.
My calculations could be incorrect, you might want to check this, and use some logic before you take that piece of advice, but do listen to me about the gear type. bevels are a pain in the ass.
Check McMaster.com for motors, gears, and everything else you will need. I would suggest zinc coated steel for the gears, but thats type I'm most familiar with. Excellent corrosion resistance, good wear resistance, and you'll get 50k cycles easy from them.