Mechanical components I
Here is an overview of the mechanical components that I used for the platform:

Rack-shelf metal is great for this type of experiment. You can bolt it together easily, make lots of adjustments, and with some cross-beams, a very strong structure can be build. The even spaced holes let you easily make accurate symmetrical constructions.
I used 30x50mm, 2mm thickness material. It was quite cheap in Taiwan, about US$ 1 per meter. You can get it up to 2.5m long.  In Europe, check for "dexion slotted angle"

For the cables I used 3mm steel cable, that easily holds the total platform weight of about 100kg (including me). Cable tensioners and fasteners are standard hardware stuff.

The gears are two sprocket wheels on 17mm steel shaft (14 teeth and 39 teeth coupled together) This was industry stuff, that could be custom made in Taiwan. The chain is also industry stuff, a little heavier than normal bicycle chain, but lighter than motor cycle chain. Since the chain has to hold the platform as well, it needs to have a certain strength. I felt safe with this type, but would have some doubts with normal bicycle chain. You should check the breaking strength.  

Update above: The chain-sprocket wheel drive generates quite some noise. A much more silent drive can be obtained by using belt-pulley. I used "A" type belt, 2" pulley on the motor that drives a 6" pulley mounted on the drive axis via #22 belt. Then a 2" pulley on the same drive axis moves a 2nd belt that is connected to the cable.  The mechanical drawings of the pulleys are shown on the side. I had these made in Taiwan, hence the Chinese comments.

The motors are real industry DC servo motors. I was lucky enough to find them on the junkyard one day. They are specified as:
Typical power 230W, current 4.4A, 2100rpm@ 52V, torque 1.05Nm. I used this motor with +/- 24V drive voltage. When I increased the voltage to +/- 36V, the platform drive forces got somewhat scary. These motors could rip the whole structure apart. 
You really can get away with much less, depending on the gearing you want to implement. Some alternatives could be electric gate openers, electric screwdrivers, car wiper motors.  

The roll-bearing wheels came from old production line conveyer belt rollers. They are aluminum with V-groove, and inside have double ball bearings with 15mm inner shaft.

For the end-stop switches I used micro-switches, with extended lever, that is pulled down by a rope and a spring.