nice and simple roger.
there are heaps of pwm motor speed controllers available on ebay as well as other electronic suppliers. this is one I found.
http://www.ebay.com.au/itm/HLDR4-PWM...ht_3419wt_1139
this one is bi-directional
http://www.oceancontrols.com.au/KIT-166.html
as for the wheels, you could use two hole saws of two diameters to drill out two cores from a ply piece. just glue them together once aligned. very simple source of wheels.
the hole saw kits are available in bunnings for 15 odd, so use say a 20mm first and take out the core, then use a 34mm and take out the core. align the two and use liquid nails. that will give you a wheel with an inner flange. just use the non threaded part of an m6 bolt as the wheel axle.
also, if you tie the thread at both ends with just a few turns on your motor shaft for grip, that will allow it to move in both directions. something like a scanner assembly. you can even spring load it at the ends.
if you're handy with electronics and soldering, you could easily rig up a 555
based variable oscillator, use this stepper controller that can do 1/16 microstepping, and get a stepper motor. that will get the movement slow and smooth.
http://littlebirdelectronics.com/pro...age-regulators
http://www.robotgear.com.au/Product.aspx/Details/459
http://www.ebay.com.au/itm/Signal-Ge...ht_2556wt_1185
secondly, if you fit a second motor under the camera and manage to get it very slow, you could pan as well.