I'm thinking about getting a 24" Cinema Display from eBay, can you run two of them on an 11" Mac Air 2012 with the connectors it has (if needed in the future)? Or only the newer Thunderbolt displays?
The 11" Air has only one Thunderbolt/mDP socket so it can only handle one mDP display. It can drive dual Thunderbolt displays in daisy-chain but not dual mDP.
It's possible to do dual displays without Thunderbolt; you could use one ACD display and then one DVI using a USB>DVI converter for example. There are also USB to DisplayPort converters but they are so expensive that I would not consider those unless you happen to already have the 24" ACDs and are just trying to find a way to make use of them. You could easily buy a USB 3 to DVI converter and a DVI monitor for much less.