How much are you willing to spend?"...choose motors that don't use spur gears. In a high-torque situation, it is very easy to break the teeth off the gears, and destroy the gearbox (which may or may not be easy to repair - or if so, may or may not be inexpensive!). ..."
"...it might be cheaper to purchase a linear actuator)....
"...if you can find it, go for a gear motor that uses a worm gear to a spur gear (a windshield wiper motor or a window actuator motor from a car are low cost examples).
For my situation (which I am still working on), I ended up building a homebrew linear actuator mechanism using a 7.2 VDC cordless drill motor/gearbox with a piece of all-thread as my lead screw. A couple of nuts with a bolt JB-Welded to it form the travelling nut (if the JB-Weld breaks, then I'll try a real weld job instead). It should be a cheap (the drill I got at Goodwill for a couple of bucks; the rest I spent less than 10 bucks on) but powerful form of a linear actuator, as well as having much faster movement than a standard linear actuator (1-2 inches per second seems reasonably possible from my testing).
Windshield wiper motors modified to act as a servo seems to be popular with stearing setups.