Eric, I haven't built one, but I'd be fairly confident in saying don't bother. a surface grinder has to have very precise bearings, better part of a G note or more and a precision balanced spindle. without this, you're not going to get a good finish. think about whats going on. depth of cut is measured in tenths so acceptable run out has to be a fraction of a tenth or it'll show up in the surface finish. and its dynamic run out spinning a heavy wheel at 3k. you need very high class bearings and a very rigid structure to make that happen. then start worrying about ways. you'll need to know how to scrape (with hardened steel strips over the scraped surface) and make hardened cylindrical rollers.
I don't think you could build one, at least a high quality one, for twice what you could buy a used one for.
they are great for making your tooling, I use basic mild steel and send it out for a deep case. nice. but do we really need hardened tooling on the home shop? not a necessity. I've made my tooling mostly, so my SG hardly ever gets turned on. because our uses don't demand the same wear resistance we can achieve the same results using soft steel and techniques like scraping
and they are dirty. if you grind dry you need to wear a mask and if you grind wet you'll taste all night whatever chemicals are in the coolant.
talk you out of it yet?
imo what hsm's do overlook and what makes a major contribution to work quality is a tool and cutter grinder. Having sharp drills, end mills and precision ground threading tools is a big step up. having both, i would recommend getting a t&c grinder before a surface grinder...it gets use more often and has become indispensable - never cut with a dull cutter again! a surface grinder is one of the tools to add when a perfect one comes along and you're feeling flush, not something to go out shopping for and not reasonable as a diy project, imo