I also fool with older motorcycles-Yamaha1980 SR500 now- and bicycles.
The above advice-buy a used modern bike-save maybe 70% on the new price-is excellent, and best bang for the buck.You can buy a $1500 bike for $500-$600-it will be maybe 5 years old.

If you decide to go 1970's 1980's old- make sure the frame is double butted Chrome Moly(or Reynolds 531 853 etc )-no point in putting nice components on a so so frame(silk purse sows ear).
If you decide to go 90's- get a good aluminum frame-save 2 lbs over steel, or maybe a 853 Reynolds DB steel.etty sure the aluminum frames are also butted on either end just like steel.

Just don't buy a cheapo frame-since better frames, on old bikes, are just a little bit more $$ .Make sure the Nishiki has a Double butted frame- not straight gauge steel, even if it is chrome moly.

Good rule of thumb- a bike with a good frame has good components.
Charlie