80mm f:4 Rodenstock Rodagon , very sharp.
I have seen these lenses for around $150 on eBay but if you can stretch the budget they try to get a Rodagon N APO, 8 element lens. This is what I use for my 6x6, the APO type is really not that necessary for B&W and you can not really tell the difference just looking at the print under around 30x40cm in size, I used to do a lot of colour printing with RA4 and Cibachrome and the APO designated lenses really make a difference with that.
For very large prints I also have a Rodagon G ( 80mm and 150mm ) witch are optimized for large printing. Unfortunately the 150mm is not much good for prints under around 1 meter on the longest side
Truth is though you probably will not see any difference between all the lenses mentioned but it is always possible to get a bad lens of any brand.
Condition is the most important thing , a dust spec or two is not a problem but if there is any sort of fungus it is best to probably forget about that lens unless you are very confident you can clean it or don't mind getting it professionally cleaned but if it is very bad the lens may be ruined as the fungus does etch into the lens elements.
Schneider also make excellent lenses such as the 6 element Componon, while I have never had one for 6x6 I do use one for large format and it is a beautiful lens with he most beautiful aperture blades I have ever seen on a lens.