You will need a hydraulic pump motor, compass, display head and computer. Maybe a rudder reference unit. Then it needs to be interfaced to your GPS / Chartplotter. The hydraulic lines need to be tee'd in and bled and the power lines and circuit breakers installed. The compass needs a magnetic interference survey and alignment and dockside sea trials must be done. When installed correctly the results are fantastic. I have seen lots of autopilot problems caused by botched installations. Unless you have lots of experience in these areas I would shop for a good tech. Furuno & Garmin make great units, I rarely see a good used one. Capt. George
Auto-pilots are set up according to the type of vessel they are being installed in. If you are looking for a pre-owned autopilot, you need one for a twin engine inboard planning vessel from 30' to 40'. Obviously an auto-pilot for outboards won't have the correct set up or the correct sized pump. Same with smaller inboards or single engine applications with only one rudder. Once you find a pre-owned unit, look it up to see what the parameters are.
Make sure it is compatible with your electronics. If you have 2000 electronics, and you buy a 2010 auto-pilot, you likely will not be able to set up waypoints and routes. same goes for the reverse newer electronics and older pilot.