Have a look at the Cetrek 730. Mine has worked briliantly for the past 1000 hours or so. Not the cheapest, but the electronics are very robust and very sophisticated. You virtually get 3 APs in one, including a system which automatically adapts to the sea state.
It will splice into your existing hydraulics.
NB with a motor sailer, don't be conned into fitting the minimum autopilot power. It will give up as soon as there is any sizable sea on the quarter or off one bow.