mgsolid2010:
Am I on the right track, will a linear actuator have enough strength to be able to push down on a common door handle?
Of course. They can really powerful. A linear actuator could easily lift your door (even if it's really really heavy solid steel door: just go for a hydraulic cylinder, like they use to move the bucket of an excavator). Just get one that's got the force, displacement and speed needed for your project.