Advice on Building a Motorized Pottery Wheel

MOTORS AND DRIVES

DC
A DC motor and speed control is easiest.  For low buck, use the parts from an old motorized treadmill--this also ensures the motor and controller are matched.  Else, there are DC motors and speed controls on ebay but you'll need to match their voltage and HP.  There are two types of DC controls: SCR (silicon controlled rectifier, common and lower $), and PWM (pulse width modulation, fairly new, expensive, but better control at very low speeds).  With DC motors, torque is constant at all speeds (whereas a transmission multiples torque at low speeds), so you will need a fairly high HP motors (3/4HP-1.5HP) motor for low speed torque.  Another issue is that most _cheap_ DC motors are very high rpm—like 5000rpm.  So you’ll probably need a jackshaft or a poly-v belt drive ($) to get down to 250rpm.  If you can find a good treadmill it will have a higher torque, lower speed DC motor.  

AC w/ VFD
The AC equivalent to DC is to use a 3 phase AC motor and a Variable Frequency Drive (aka "inverter").  The drive is typically pretty expensive, tho, and has poor control at very low speeds.

AC w/ transmissions
These are appealing, as you probably know, since 120V single speed AC motors in the 1/3-1/2HP range are very common and typically $10 used.  The HP is much lower than w/ DC since the transmission multiplies the torque at low speed.
1) Multiple pulleys: as noted, only a few speeds, and a (dis)engagement system is needed.  Awkward but cheap.  4-step pulleys are at www.mcmaster.com.
2) Cone-drives: the traditional Shimpo approach, done DIY by Vince Pitelka (search for his posts on the ClayArt listserve).  There are two approaches: a) a small rubber ring on a jackshaft which then drives the wheel by a pair of pulleys;  b) a large rubber ring (12"-14") direct drive to the wheel.  The latter approach is well-suited to using a car wheel/hub/bearing (see below).
3) Zero-max: this is an industrial transmission that varies output speed from 0- 400rpm (for 1650 input).  On ebay they go for $30-$60 for one with reasonable torque capacity.  
See www.zero-max.com/products/drives/torque.asp  
Most of the zero-max drives on ebay are low torque, so you'll have to look a while.  I got a model Y41 for $40 and built my wheel using it with a 1/3HP motor.  It has 60in-lb capacity which turned out to be more than enough for me.  The downside to the Zero-max is vibration--not too bad, but not nearly as smooth as DC.

WHEELHEAD AND BEARINGS
1) Buy the Brent wheelhead with integral bearings and shaft.  By far the easiest.  $70 for the wheelhead, and $110 for the wheelhead + bearings/shaft.
2)  Wheel bearing, spindle, hub, and wheel from a junked car (fill the wheel with plaster or cement).  This is the approach of Jolyon Hofsed: www.ceramicsmonthly.org/mustreads/wheel.asp
I have not tried this "transmission", but I am guessing it would be hard to control the speed steadily.  But, if one changed the rubber plug to a cone, and then riveted some reinforced rubber tubing (split down the middle) to the inside rim of the wheel, you have a cone-drive system (option "b", above) half- way done.  Durability is a question and entirely dependent on how well the rubber can be fastened to rim.  Also need to get the contact point to the cone consistent (speed fluctuations), which might be hard.
3) Front wheel bearing/spindle/hub from a FWD/4WD car.  Mate this to a wheelhead of your choice.  The main advantage is that you can drive from below and separate the motor/tranny from the clay.  The cartridge bearing needs to be kept together (easiest with the original CV joint).  I took this approach with my Zero-max tranny, and got the car parts for free from a collision shop (unlikely to get the CV, tho).

Overall advice:
1) If I were doing it again for low $ and effort, I'd use a DC motor and controller from an old treadmill, mated to the Hofsted wheel head/bearing via belt drive.
2) For a nicer wheel at higher $, use the Brent wheelhead/bearing/shaft instead of the Hofsted design, mated to the treadmill motor via belt drive.
3) To find AC motors, car parts, etc. at low $, go to a "metal recycling" yard (or collision shop).  Avoid auto junkyards if you want to keep the cost down.  
4) For pulleys, belts, and hardware at reasonable $, go to mcmaster.com.