anything i should look in particular?
Woops, only answered one question. Brought in a '93 4020 last January. If somebody is selling it, I can guarantee there are few things wrong with it, and pretty much everything on it is cheap and easy to fix.
If the screws are loud, 99% chance its not the screws, the thrust bearings go bad with great frequency, especially if it has the water cooled screws, the seal leaks and it dumps directly into the thrust bearings. About $80 a set to replace and not very difficult, I also suggest beefing up the motor couplings while you are in there, they are really not attached to the motor or the screw very well and can be a serious source of lost motion(back lash).
Tool retention can be a problem, and a reason I think a lot of people get rid of them is busted bellevilles, $50 for the bellevilles, and $20 for the drawbar floater which can have a huge influence on tool retention.
It should be a box way machine, totally adjustable, though a bit of a PIA. The older gibs where steel with turcite, which seem to get trashed, especially the lower ones on the Y axis. 4@$45 a piece fixes that with bronze gibs.
If the toolchanger is having problems, a lot of times its just chips built up in the slides, which can lift the carousel up and cause a bunch of headaches. 2 minutes with an airhose and that is fixed.
Example, when the '93 I have came in, it had .015 of slop in the Y and .005 in the X, thrust bearings, 4 gibs, and a little bit of time tweaking it all in, and its pretty much dead nuts now, no witness marks on the quadrants, decent machine that is easy to fix, especially for the money they are going for right now.