Disco,
Funny that you ask, I just rebuilt a NH 256 (mid-70s era, I'd guess, don't know for sure) from the ground up this spring. Since it should be pretty similiar to the 56 that you are looking at, I can comment on a few things.
1) The previous posters are right, grease is the way to go for the gearbox. In fact, that is what the manual asks for (and specifically warns against gear oil).
2) The tip to check the play in the universal joints and the shaft of the gear drive (goes from the gearbox to the left wheel axle) is also a good one. I attempted to rebuild and eventually replaced with new... and it wasn't cheap.
3) Check the play in the tine-bars (the long horizontal bars that the tines are attached to) by grasping with both hands and "twisting" front-to-back. Too much play can be worrisome, since replacing the bearing assemblies on each end of the tine bar is a real pain in the neck. Believe me, I know, 5 bars x 2 bearings each means plenty of experience!
4) Check for cracks and welds in the frame up near the receiver where you hook up to the tractor drawbar. If somebody ran it into a fence you can get issues there (mine has several welds, although whoever did that work did a good job and it's solid).
5) Check for cracks in the "spider wheel" (5-spoke wheel that the tine-bars attach to). If somebody runs it too low to the ground and really pile-drives the tines into something, the force can crack that wheel. I had to replace one on mine. Later models (I'm not sure of the year) had re-enforcing steel plates on the wheel (where the tine bar attaches). You probably want to check and make sure those are there.
6) Best check that you can do just looking at it is just spin it up with your hands (make sure it's raised up high off the ground, of course). It should spin smooth with no grinding or clanking.
Oh yeah, forgot to mention: It's a great rake! Runs great and very robust. I mean, how often have you seen a design remain basically unchanged for 40 years?
Chet.
