I think many here would disagree that fit is not important. While some amount of adjustment and tweaking can make a range of sizes work, a poor fit can really hamper your ability to enjoy the ride. If I were you, I would focus on finding something close in size to the bike you have now (within 2cm in either direction) if it really fits you, and go from there. Otherwise, why bother with the time and money involved with a vintage bike?

I think that Nishiki is actually a fine platform to start tinkering and learning. If you make mistakes, no harm done. Don’t invest in new parts, just overhaul the ones already on there.