Well I was thinking an old-style rheostat dimmer, which is just a big variable resistor, which should work fine, really. I'll agree that newer kinds of dimmers don't have a chance of working. I was just trying to come up with a cheaper easier option than an entire control system. One fixed speed just seems like a bad idea. Either you'll be in a rush and it's too slow, or you'll be in traffic and it'll be too fast. At the very least, make up some voltage dividers and have 2 or 3 speeds and a multi position switch.
As to actuating the scissor lift, I'd go with a lead screw and a motor turning it. I was specifically thinking of a device the Mythbusters used in the ceiling fan episode if you've seen it. This too would be a good place to have speed control.
Finally, whatever system you use, you might want to think of a way to lock it in place. Better safe than sorry and all that.
EDIT: This is what I'm talking about
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200-Ohm 50-Watt Rheostat