The New Holland 450/451 sickle bar is a quietly brilliant design that's been around since the '60s and has stood the test of time. It is still made today I think - price has gotten pretty high. One of its best features is the pitmanless drive for the knife. The knife end is suspended by a Chebychev linkage that keeps it going back and forth in a straight line without rubbing on a wear shoe.
They have a clever three-point hitch arrangement for raising/lowering the bar. On many newer compact tractors it needs a top-link extender so it works well with the lift arms.
Some of these mowers are getting pretty old - so condition varies all over the map. NH450/451 model differences are small - straight or tapered roller bearings in the knife drive, different cutter bar lengths 7' up to 9'. Kept up, they are a great tool. But there are also some tired machines that come up for sale.
Things to check: look it all over for cracks, at the break-away links for evidence of weld repairs, verify the belt & knife mechanism turns smoothly by hand. If it's hard to turn the knife is binding somewhere - indicating work is needed. Check the serrated ledgers are in place under the sections - look for loose, missing or broken sections or excess clearance between the knife and guide plates. Good luck, Dick B