I don't claim to be an expert but I have been working on two NHs balers, a 68 and a S68 manufactured in 1958-1960. In neither case are the knotters a problem and you can still source them over here in the UK (probably a lot easier in the US). Having said that, they are expensive and replacement of a compete unit unlikely to be financially justifiable - walk away and find another example.
3 areas have gone on these old balers:
1. The wooden runners (aka bearings) that the plunger sides on in theses two models. Most later ones use conventional roller bearings. Having said that I just made a new set of runners myself from a chunk of English Oak - total outlay zero. The cost of replacing roller bearings will be much higher and they may be difficult to find.
2. The tin work. This is 60 years old, it has been rained on, stored outside or brought damp into a shed and stored inside in which case it rusts faster!. You need to look very carefully. seems anything below the waistline (as it were) is suspect (so you have to crawl around underneath). 12 of 13 pickup guards were rusted to varying degrees - not worth replacing (at about UKú25 each) but actually easily fixable with some aluminium sheet for almost no cost. The base of the feeder chamber and the base of the twine chamber were both corroded. Fortunately these are basically flat panels. I one case I'm patching with 2mm sheet, in the other fabricating a new panel. With basically flat panels no sweat, anything complex and you will have to be a metalworking guru and dab hand with a welder! Needless to say, NONE of the tinwork is now available as spares (and is prohibitively expensive). Another critical area is the underside of the plunger chamber. The plunger chamber and chute are a single welded assembly and the base seems to rust (not unsurprisingly). This is pretty substantial metal so a decent welder would be able to fix it. Also have problems with the needle pivot yoke where it pivots on the sides of the chute - surrounding metal rusted through. Still trying to decide if that is easily fixable. My sense is that noticeable rust in critical high load areas on the chamber and chute is a reason to walk away.
3. Bearings. this probably depends on previous greasing. Most of mine are pretty reasonable but difficult and expensive to replace unless you can find alternative parts from a specialist bearing supplier. I have replaced the tine bar roller bearings to eliminate slop and noise, but that was inexpensive. Same goes for chains and sprockets - if they have been well greased should be ok, otherwise expensive and hard to source.
My sense is that things like haydog springs will always have to be replaced, along with some pick up tines - but these need to be regarded as consumables and appear to be common across multiple models.
Bottom line, be very sure before messing with the knotters. You can see the shape they are in, they can be adjusted and appear pretty robust. They are also on the top out of the crap in the hay field. They either tie or not and knot failures are just as likely to be due to the tine bars not packing the hay into the chamber evenly, the plunger out of alignment, the knives blunt etc. The real problems tend to lurk out of sight.
Obviously newer is better than older (easier parts availability), but condition is key. Looking at the parts books the design of these balers has remained pretty consistent over the decades (i wouldn't want hydraulics for bale tension etc) which indicates a sound design, but they are all getting old now and will increasingly need TLC and a willingness and ability to fix things that cannot be replaced.
On a more general note it would be good to hear from others about common failure areas they have found across the makes and models and the relative problem of fixing them. I have started a new thread to collect this.