80's Japanese bikes like Miyata, Nishiki, Panasonic, Bridgestone are undervalued right now, except for the high end stuff. I snagged a nice Miyata with butted chromoly, complete bike, for $40 for a friend.
Horizontal dropouts stayed around till the mid-90s; 700c wheels became common in the early 80's so you've got a good time window of quality frames with both.

Garage sales are your best bet for an inexpensive bike. Or thrift stores if you live in a large city and can visit the stores on a daily basis to find a bike. Flippers lurk everywhere to snag the good deals then throw them up on CL or eBay so you have to act fast.

Simply convert the 56cm to inches(divide by 2.54).

Italian bottom brackets are still being made; it's the French bottom bracket and the Raleigh 26tpi bottom brackets you got to be careful with. If you've got the original bottom bracket cups and spindle you can make it work, if not, a replacement is hard to find.

Look for dropouts that are forged or at least stamped from heavy gauge steel. Dropouts held in place by crimping the stays I don't mess with but I see plenty of people riding on them.