I've got a New Holland baler, I think it's a 278. The advice mentioned earlier is excellent. One thing to keep in mind is that if you get it for a good price, and it's running, anything that goes wrong won't be too hard to fix. In my case, I'm not that mechanical, but I have a good farm mechanic. When I got my baler, the knives were pretty dull, and some of the fingers were broken. I ordered new ones, and my farm mechanic had it all set in about 1/2 hour, so it wasn't a very expensive fix. Parts are available for all the New Holland balers at NH dealers.
The main thing I would do, is have the seller hook it up to a tractor so you can see it run. In my area, the vast majority of balers you see are NH, and all my full time farmer friends swear by them. I've only used mine last year, but so far, I'm very happy with it.