This thread fits my situation as well - I'm refurbishing a 28' Checkmate with twin Bravos and watching the prices versus my pocketbook and would love to piece together my own system to save a few grand.

The plus to buying a new kit is that everything is designed to work together -
-the size of the helm to match the rams,
-the length, throw and pressure requirements of the rams
and the type of helm "valve" and where to get the steering wheel adapter are worrisome. I'm thinking to maybe go with an add-on external only until I figure out or find the internals. I do know I can't spring $4Gs for a Mayfair anytime soon...

Early mercury external steering used Corvette rams and even the control unit off the steering linkage which went on the tiller arm.