I guess it all depends on how long you want to wait, and what is an acceptable condition. If you are using it to generate income, then time is money and if something breaks and you can't get replacement parts you are toast. If you have a big old machine, they are often hard to sell, individuals are typically not looking for a 6000 lb machine, and businesses are looking at newer and often CNC type machines. The era of good used iron is drying up, mostly because industry for the most part no longer uses regular mills and lathes. Machine shops that are closing or selling off machines have often had a hard life an no longer meet specs. When I looked at knee mills in the Tucson area about 8 years ago, they were pretty beaten up and cost difference with a new machine just didn't make sense to go used. Also parts were an issue and very costly. I was more interested in using my machines.
As far as lathe, my recommendation is going with a 1640TL and do a basic VFD install. You will get much better longevity and the resale value if you decide to go CNC, although I still think there is a role for manual. I have the ERL-1340, have also used the 1640 model and they are killer lathes, very good quality and solid.