Never specify an autopilot 'just shy' of the displacement weight of a boat, if cruising you will probably add another ton to this. Further, if you are over say 75% of the capacity of a particular drive system for a boat, specify the next size up.
I think this should emphasized.
When using an autopilot it is easy to overlook balancing the sails. When manually steering the boat you can feel 'weather helm' due to unbalance sails etc. and you can make adjustments, the autopilot won't, it will try and keep you on course possibly overloading the capacity of the drive.
In our case we balance the helm before enabling the auto pilot and whenever there is a wind shift. In fact, with the pilot steering, one has plenty of time to balace the sails even if only one person is standing watch.
The mounting and geometry is very important to the performance and life expectancy of the drive. If you are lucky you will have a convenient surface to mount the static part of the drive to. If not a platform or shelf will have to constructed of sufficient strength to put the drive in alignment and to absorb the transferred load.
The majority of the system failures I have seen have either been with the electrics controlling the system or a failure of the drive mounting. This is certainly one case when bigger/stronger is better.
Also do not consider attaching the drive/ram to the existing quatrant of steering system. You need to have an autopilot tiller arm mounted to the rudder stock either a custom manufactured on or one similar to those supplied by Edson.
I think this is case dependant. A separate tiller arm certainly has its merits but I think it can also be very problematic. Given its construction, I cannot imagine the quadrant on our yacht failing but we also have an exceptionally well built boat.
I will reserve this post for the drive mechanics. This will be the most complicated part of the installation and unless you are proficient in this type of installation I suggest you seek assistancew from someone with experience.
On this I have to say that the hydraulic drive assembly on HyLyte is very robust and very simple: