I am new to vintage and road bikes, so forgive my ignorance. Having cracked a mountain frame, I am uneasy about an old frame that I don't understand. (current commuter=lifetime warranty; I get that...)

I am looking at mid-level, Japanese, 80's road/tour bikes, which seem affordable, and easy to find, fix or upgrade components(yes/no?)(Unlike maybe a Puegot?). Are they reasonably fast; enough to hang with the "C" group, 12-17 mph, I assume? I can pretty much do that on my tank commuter.

These bikes have frames like Champion 5 chromoly, and Fuji chromoly 441. What is hi-ten, chromoly, lugs, butted...? It's all steel to me. If I can't afford hand-made Italian, what else should I be looking for, if not Japanese? Can these kind of frames support my wookie-like frame? Will a tall frame automatically handle a Clyde?(was 330, still a clyde at 210...) Any suggestions on what to look for in steel frames, how to rate them, links, etc.? More general tips on a starter vintage road/tour also accepted and appreciated; does not have to be Japanese. thanks