What should I look for to see if my older furnace will accept central air or if I need to replace it

Does your unit vent with plastic or metal pipe?

What you are looking for is. What does your t-stat wire conect to. If the go to a control board you will be fine.

It has a metal vent and the thermostat wires down to like a small transformer and a relay in the furnace. I was also thinking about buying a used furnace but wanted to make sure it worked with a/c, so as long as it has a electronic control board should it work

Yes it will if it has a control board. Does your furnace now have more the 2 terminals on that relay or is it just r and c?

It looks like the thermo wires go down and wire into the gas valve and the other into the fan limit switch. I was also wondering my furnace is probably vintage 1983, and is 80,000 BTU would I be able to get a 75,000 BTU and be OK

Yes that should be fine from 80 to 75 no problem.