I haven't dug into the doors on an S14, but usually in my experience when lock gets sticky usually the power actuator is at fault. The manual locks on S-chassis cars are very simple. Just a few bars connecting the door latch to the handles, so there's not a lot of room for things to foul up. Clean out the door latch and lube it, and if that doesn't fix the issue, make sure the power actuator your friend installed isn't the culprit.
As far as what years you can get pieces from, they should be the same from 95-98. If you try to land OE power equipment, try your best to get 96 as there are electrical differences year to year in the second gen. If you want to go with OE, grab a service manual from our online library
http://www.nicoclub.com/FSM
Keep an eye on our classifieds for S14 part outs if you end up needing to buy pieces.
And welcome to NICO