We are in the same position (1/2" hex shaft work, only without the CNCs lol) and I've been searching Craigslist for four months now without finding any particularly good deals. Knowing your budget would be a big help here; I've been looking in the sub-$1500 range, but if you have $3-4k to drop on it it's considerably easier to find a good one. For example,
this one appears to be in good condition right now, although you would still need to check it.
I would say be wary of most flat bed lathes- they seem to have problems holding tolerances, especially on the tailstock. This includes the Atlas lathes; even though many people really like them it's hit-and-miss, especially if you don't have a machinist mentor to help you check the tolerances. Although many old lathes are good, many of them are also bad, and without thoroughly checking all the tolerances on it you can't be sure that you're getting a good machine.
The Seig lathes, although made in China, are able to get good results (see
this site) and for just working on 1/2" hex shaft it seems like a good deal. A very highly-reviewed lathe in the same size class is the
Micromark series of lathes, which is nice if you don't fully trust the Seig line. They also have the handy cam-lock tailstock. Little Machine Shop also sells some good tools and a few machines.
You may also want to outfit the lathe with a 5c collet adapter (preferably a tru-set) so that your 1/2" hex stock runs true in a collet, which can run you $300 or more. This is so that less experienced students don't have to rely on a 3-jaw chuck and their iffy runout to drill holes and bore on-center.
Another extremely handy accessory would be a DRO, and if you have the funds definitely go for that. It makes anything with circlips 10x easier. Speaking of circlips, getting something to make very small grooves is hard to find. The Nikole Mini-Systems grooving tools are the best I've found so far, that or just grinding down a parting tool blade to the right width for your groove.