My goal is to add a 5rpm DC motor to drive a full length piece of all-thread to move the carriage for time lapses - but have not found any locally yet, so after I figure out the linkages eBay has a few to choose from and we'll give that a go.
The shopping list (save for the wheels) is a Home Depot/Lowes/Ace Hardware trip away, and approx. $15.00 of parts/pieces.
2 - U-Post fence posts (3')
1 - 1/4" All Thread (3')
1- 1/2" square steel tubing (3')
1 - Simpson T-Strap (6")
4 - 1/4" Cap Nuts
12 - 1/4" Washers
3 - Inline Skate Wheel Assemblies
(In the picture are some 3" x 1/4" bolts and coupling nuts, these are/were to make leveling legs but so far have not been needed)
UPDATE: The motorizing of the dolly is complete, the instructable for it is here: http://www.instructables.com/id/Yes-Another-Camera-Slider-NOW-MOTORIZED/
Step 1: Dolly Assembly
The T-Strap is rigid enough, dirt cheap, and has perfect clearances for the wheel diameter - the mounting holes are large enough to allow the wheels to move in and out 1/8" or so for fine tuning tension in the frame.
A couple washers help take up the space the skate frame provided, and the bearings and axles all go together as easy as could possibly be.