I would skip the crimper. They are a pain to use and you will find your sickle bar mower will not work well in good alfalfa or even heavy clover. Also you want to crimp the hay as quick as you can. This is to avoid leaf loss and speed drying.
Your NH 451 maybe setup to pull a crimper. Some of them had the 540 shaft already on the back and you just bought the hitch. (See picture)
A good model to look for is a NH 488(9 ft) or if you need a smaller machine go with a NH 478(7 ft).
I am a JD person but in a used a mower conditioner I would go with a NH over a JD. The 1209 and 1219 can be a expensive piece of equipment to repair if they have been abused too much in the past. They also are heavier and pull harder.
Also the NH 488 has been in production for almost thirty years. So they must be a good machine to keep selling like they do.