I agree with rbergeron that the New Holland 451 is the best choice because the knife drive (the most troublesome and expensive part of the mower) is simple and easy to check for condition. To give credit where it is due, the knife drive mechanism was actually invented in the mid 1800's by a Russian mathematician named Pufnuttii Chebyshev who was looking for an arrangement of arms and levers that would convert rotary motion to straight line motion without slides, cross-heads, etc.
I first experienced the NH drive on the farm many years ago and when I needed a sickle mower for some rural land I bought I first looked at the NH. I was turned off by the price of a new one ($8,000) and the attitude of the nearest NH dealer. So I looked quite a while for a used one, but the local ones (Craig's List and the State Farmers Monthly) were snapped up pretty quick. I talked to Mr. Sweet, but he couldn't seem to keep one in stock long enough for me to buy it.
Then a John Deere 350 came up nearby on EBay and I bought it. The JD was my second choice because the knife drive mechanism (known as a "wobble box" because of the motion of the principal part) is more complicated than the NH, more difficult to inspect without a teardown, and has four proprietary parts vs one (or maybe two) for the NH.
I got lucky and the JD turned out to be a good buy ($2,000). JD still carries every part for it, and my local JD dealer is very good to work with. So I am satisfied with the JD 350 (so much that I bought a second one from the same seller).
I dissasembled the JD wobble box and inspected it carefully. I am satisfied that with proper maintenance (greasing) the proprietary parts will outlast me.
If defense of JD (and International Harvester, Massey Ferguson, and some others) who use the wobble box design, the complexity allows it, in theory, to eliminate all vibration, while the NH design is like a one cylinder engine, the very best you can do is size the counterweights so the vibration is evenly divided between the vertical and horizontal planes.
Prices are crazy. If a knowledgeable seller is involved, a good NH451 or JD 350 will bring $1,500-$2,000. But you occasionally see one for half that, and those usually sell very quickly.
Finally, I bought the three point hitch (JD 350; NH 451) vs the pull type (JD 450,; NH 456?) because I like the increased maneuverabily. The disadvantage of the TPH models are greater time to hook up and significant offset weight on the tractor. That should not be a problem for a MF 135, but I don't think a compact utility tractor has enough weight or strength to deal with the offset load.
