The actuator needs a rebuild or can I buy a new one?
This depends on which one you have, Merc and OMC used an actuator with a rebuildable control valve up to about 1989, after that the valve changed. The cylinder isn't rebuildable as far as I know but that old style control valve can be rebuilt. Why? Well it's the same one used in old style Corvettes and Mustangs, I think they were made by Bendix and there are rebuild kits available.
I have a NOS control valve for my Cobra and a cylinder as well. I saved those for the future and installed a nice used unit that is working fine.
If you have a newer than 1990 transom mount, I'm not sure what your options are, those actuators aren't cheap, close to a grand for a new one.
The worst thing about that job is the cotter pins that OMC and Volvo use to retain the mounting bolts, it was hell on wheels to get them out. After that, not bad. Merc uses a locking tab, might be easier.

