If possible, try to find a copy of CS6, that way you'll have a little more latitude with opening the older files. I believe CS5 was the first 64 bit program. Speed is worth it...I wouldn't go back any further than that (CS5). When the Creative Cloud took over, there was a period of about 6 months where you could still purchase the stand alone CS software. That is no longer the case.
"Back in the day" you could get older Adobe software fairly cheap on ebay. That's obviously changed since then.
JB