One thing I can tell you is there is a major advantage to having an all
electric autopilot. Now you can couple it to the VOR head and the GPS
(if you have one). But not the DG. If you couple it to the DG and the
vacuum fails then it goes into failure mode. Some people say that they
wouldn't have an autopilot without coupling to the DG, and it is nice.
But consider this. You are flying along in IMC. You loose your vacuum.
You still have your autopilot, fully functional. Works like it always
did. Most all of the all electric autopilots have an "uncouple" feature
where it just flies wings level (off of the TC). Sort of poor mans
couple to the DG, as you just turn the airplane to that heading. It
tracks the heading pretty well. So if you need to track a heading just
use it uncoupled in wing leveler mode.

If you must couple to the DG, go ahead. Just make sure it is all
electric otherwise. No coupling to the AI or using vacuum to power any
of the autopilot features. But really, you don't need DG coupling. It
makes failure modes MUCH more complicated.

Having an autopilot coupled to a GPS is the cat's pajamas. Right on
track and it's an accurate track from anywhere to anywhere. If you
don't have a GPS that it can couple to now, just make sure it can in
the future if you upgrade to a GPS. Usually it's an IFR GPS that it
couples to. I've never seen coupling to a VFR GPS, though I suppose
it's possible.

I think most of us would do just fine with a coupled autopilot and
altitude hold. Anything more in a small GA plane is overkill, IMO. The
times you need it is on long cruise. No problem flying climbouts and
approaches by hand. If you can couple to the VOR head, then you can let
the autopilot fly the ILS right to left and you manage the descent
rate. That is what I do. Works well, though I never use it in IMC as I
always need the practice.

Anyway that is my take. I have a Century I coupled to either my VOR
head or my IFR GPS. No altitude hold in my bird though I would like
that. The plane flies pretty well with just the trim, not a lot of
altitude adjustments necessary, at least in calm air. But altitude hold
would be nice, if I were putting one in now, I'd get it.

But even just a simple wing leveler setup works pretty good (no
coupling at all). There are lots of options. But if you fly in IMC
consider making everything electric. Vacuum failures are fairly common.
Electric failures, at least in fairly new airplanes are rare from what
I have seen.