I have two Sheldons, a 10' and a 13". Both are excellent lathes.
The only complaint I have is with the 13" which is the older one and dates back to the '40s or '50s: The cross-feed and compound dials are very small; it is very difficult to split a thou, but I do it routinely.
If you have a chance to buy a well-cared for Sheldon, go for it!
There can be hidden defects in the best of looking lathes, however. My 10" had been run in back gear without adequate lube between the belt pulley and spindle. The spindle was badly scored. When run in direct drive the pulley would pivot on the direct-drive locking pin, causing two distinct knocking noises with every revolution.
I know of at least one other person who found a similar problem in a Sheldon. As for mine, I had to grind the scoring off the spindle, then bore and sleeve the pulley. It was an inexpensive but time-consuming repair job.
Be sure to check the spindle for runout. If the lathe you are looking at has tapered roller spindle bearings, it should have barely detectable runout as checked on the Morse taper. If you see anything more than a ten-thousandth, you may need to replace the bearings.
My 10" had three tenths runout and I had to replace the bearings at a cost of over $400. As it turns out, in a previous lifetime someone must of gotten tired of filling the oil reservoirs and packed the bearings with grease. Big mistake.
My main frustration with that repair was trying to get the bearing dealer to understand that off-the-shelf run-of-the-mill bearings would not work. The dealer didn't know that there are different classes of precision, even though the bearings carried the same part number. He didn't know that the precision class is etched on the edge of the race.
I like my Sheldons.
So many projects. So little time.