As I have said before, I think it well worth the money to buy a course computer with gyros. You will really appreciate it downwind. I'm not sure there are huge differences at least to the first order between brands. If your drive is a Raymarine linear and still works, you can easily interface a new course computer to it, they are stone axe simple devices. If it is a wheelpilot type drive, it is probably going to be inadequate for that size boat regardless of what is controlling it. If your existing drive will work, you should be able to install a new computer and control head for under $2000.

In my opinion a reliable below decks always-ready-to-go-just-hit-the-button autopilot is one of the very best investments in a sailboat. No levers, clutches, widgets to deal with, just hit "AUTO" and deal with whatever else you want. More useful than the engine, in many instances.

I have been happy with the Raymarine. The X series much more so than two generations back, before the gyros. My X30 seems to be able to steer the boat in the worst conditions without significant course deviations. I have no reason to believe that a B&G or NKE wouldn't be just as good or better (but they are more expensive). Personally I would stay away from hydraulic - your size boat does not require it, it WILL someday leak, and it adds drag to the steering when it is off.

If you are at all handy with tools, installation of an autopilot is pretty easy. The hardest part is mounting the drive and tiller arm. If you already have a tiller arm and drive, that should make it simple.