I just finished a Si-Tek install in the Spring. It's totally possible to do it yourself. Can you do it? Only you can answer that. It certainly wasn't the easiest thing I have done. There were lots of wires to run, hydraulic hoses to measure and have made, and a lot of gotchas along the way. There are easier ways than the way I did it to install one, but I like to make sure I think it through and try and minimize future problems.
I can post a few pics of the finished product if you would like. Chuck at Trawler Beach House has a pretty good write-up about installing his Garmin. Do your research though. I would tip my hat to getting a Si-Tex (rebranded ComNav unit) system. It's about half the price of the Garmin and Raymarine units. My SP-70 is at the end of its development cycle and ALL the kinks have been worked out. They are about bulletproof. Nor does it belong to the software-gimmick-of-the-year club. It does two things and two things ONLY. Pilots a straight course or follow a navigation course via NMEA1083 from my chartplotter. I can't do figure eight's in the water, conduct search patterns, or write my name in the river ;-)
Anyway, you need to be pretty good and several things to do an auto pilot install. Seeing that you didn't touch on your skill set, I can't say if YOU are capable of it, but I can say that I did it... and I would guess that many other here did it too. Go look at the online manual for the unit you are interested in and give it a read. You'll know quickly if you can or not.
Tom-