Before you get too far in, assuming what you have is complete, look into the cost of fittings and hardware and other little bits. Even at full price, the Harbor Freight air/hydraulic jack "might" be pretty competitively priced as that stuff adds up FAST. Plus it makes a very clean, self contained installation. Plus you retain the manual lever when you want to "feel" what you're pushing on. That's a very valuable feature. Last weekend for example, patching up the decrepet old mower deck again, pushing a pin into a blind hole with a thin bottom, and pushing a bearing up against a snap ring on a spindle. The press is quite capable of obliterating either of those jobs with very little notice.
A second thought is "what is your use going to be". Just to consider, as your use might be very different from mine. I took my air/hydraulic jack back out to use as an actual jack, and went back to the original manual jack on the press. By the time I drag the air hose over, and push one little thing together or apart... It's faster to do it myself. I'm not saying this will be the case for you, but just consider it. If you use it a lot, it might make sense. For me it did not.
Outside of those two thoughts... It sounds like a cool project if you've got a plan, and it sounds like you do. I'd love to see the pictures.