Couple of thoughts:
Trim limit - use a pressure relief valve. You stop trimming when it changes note, and removes more electrics that will corrode & go wrong. (and you need the relieve valve on the down side in case you hit an underwater obstruction anyway)
Merc legs for inboards have a standalone motor / pump unit to drive the cyls on the transom. As you are building from scratch, a couple of lengths of hydraulic hose and you can mount it on your transom or under your seat.
While we're on the subject of off the shelf hydraulics, I refuse to believe there isn't a single ram out there already......
Is your swivel bracket man enough for what you are about to ask it to do? - Remember a manual tilt engine the bottom of the swivel bracket rests against the trim pin. Smaller engines are usually designed to be as light as possiible. By going PT (I assume you are planning on fitting the ram where the damper now sits) if there isn't a PT version, you will be putting stresses into the bracket that it was never designed to take. (if there already is a PT option that's not an issue)
For powering it, have a look around the breakers - the Merc units are quite modular - mine is powered by the motor off an Alpha drive leg, but if you are planning on using an "off the shelf" pump, a car wiper motor might work?
Shame you didn't ask this a year ago!