Wether balers of this type are any good is now largely due to how much work theyve done, how well maintained and wether theyve been kept dry. I think a 68 will date from the late fiftyies or sixties so at best its at least 50 years old ! Ive used one very successfully in the recent past, had problem with knotters, but it was down to wear. The knotters are more or less the same on most balers....damn clever chap that invented them !
I currently have a new holland 276...do just a few acres each year but have had no real problems other than the PTO shaft coming loose a coiuple of times: oh yes, and it stopped knotting on one of the knotters last year. After a short period of panic i thought what would i do with my computer, so i removed the strings gave the knotter a good clean, re strung it and off we went. (just like turning your computer off and on again !) After chatting to my farmer mates they said it was most likely the once in a blue moon event where the knot you tie to join the rolls of baler twine together with happens to be in the knotting mechanism when it tries to tie the knot.
As others have said most of the wearing parts seem to be available either from the manufacturer or from places like balerparts.co.uk (some name like that anyway)
Good luck, it is very satisfying doing your own baling, a good tip is to get friendly with someone else near you who also has one, then if you have a major break down half way through your crop, you can get their help, you can offer the same service......specially if rains on the way !