I'm considering a Clausing 6903 lathe. I know it is a varispeed and there is a bushing in the varidrive that has to be in good shape or things get expensive, the power feeds work off the lead screw instead of a separate shaft. (not sure the implications there)
I would welcome informed comments on the pro/cons of this lathe, things to look out for, a link to a scanned manual, ect.
I've got a Clausing 5418 -- about the same time as the 6900 series I think, but step pulley instead of variable speed pulleys.
The power feed working off the leadscrew is still a good thing, because it eliminates the wear on the threads of the leadscrew when you are just using feed not thread cutting. The drive is taken from a keyway milled the length of the leadscrew, and is shuffled through gears in the apron to select either longitudinal feed or cross feed.
Mine came with a bed turret, and the threading dial was unused in a drawer in the pedestal, because all threading was being done with Geometric style die heads, so there was never any need to engage the half-nuts. As a result, while the cross-feed leadscrew was worn to the point where it presented a sharp crest on the thread, there was no wear on the longitudinal leadscrew or the half-nuts.
I got a normal tailstock for it, and swap them back and forth depending on what I am doing with it.
I would consider any of the Clausing machines of this period to be excellent machines.
As for a manual -- Clausing Service Center will supply you with a photocopy matched to your serial number for a quite reasonable fee. Just call them with the serial number and the model number, and talk to them. And some parts are still quite reasonable -- while others, which have had to be made after they ran out of old stock, are rather steep, but still available. Yet other parts are just unobtanium, but those don't tend to be serious problem parts normally.