I dont think I would waste my time. Surface grinders are all about accuracy and the chances of building a decent one are not so great.
Then there is the spindle... You wont get a decent finish unless you use the right bearings and those can be several hundred bucks a piece.
A well outfitted used surface grinder can be found for pretty cheap. $1000 or less. You will end up spending that much buying parts alone to build one.