For my project, I have to build a model, probably around 1:20 scale, so I am thinking of linear actuators, or "home made actuators" build to resemble the large hydraulic ones. ( a double cylinder, with a spring inside it, and a threat connecting the spring to a stepper motor to control the force compressing it.)
You'd probably be better off using threaded rod and a captured nut; see here for a small example, which can be scaled up:http://blog.davidjbarnes.com/2009/03/making-linear-actuator-for-robots.html
...also, do a little bit more googling and research on your own; many people have made similar actuators (also, something similar to them have been used for low-cost homemade CNC machines to move the X/Y/Z axes). Such homemade systems won't stand up to heavy usage (such as in a full-sized implementation of your system), but for smaller loads that don't require a lot of accuracy, they work fairly well.
If you need to track position (and/or end-stops), that will add complexity and cost, of course. The driving motor can be nearly anything, of course. You can do direct-drive if you have a powerful enough motor, but most designs (including commercial ones) use belt, chain, or gear drives, indirectly off the motor with the motor offset, or planetary gear drives with the motor in-line with the shaft. Also, commercial linear actuators do not use regular "all-thread" cheap threaded rod (like you would find in a hardware store or the like); they instead use ACME threaded rod, or on the more expensive actuators, "ball-threaded" rod (my terminology may be off on that last one; I can't remember what it is technically called, but the "traveling nut" uses ball bearings to reduce friction, and both of those have limited lash-back).
Finally, to also help your searching: Hydraulic (and pneumatic) cylinders, while technically being "linear actuators", aren't typically referred to as "linear actuators"; what is typically meant by "linear actuator" in most cases means "electrically actuated", typically via a motor of some sort (and generally a motor that rotates - if you dig deep enough, you'll run across linear actuator "solutions" that use linear motors, which act/work kinda like a stepper motor that has been "unrolled"; these kinds of actuators will not work for your application - besides which, they rarely are inexpensive).