Llike Don said, you'll know when you look one over. I've had a bunch of them, and a couple I knew I didn't want the minute I got them.
I had a '49 that appeard to be complete, just a dirty old tractor. I got it running and it ran "ok" but was no champ. Then I started looking around, cleaning, etc., and everything I touched on it started to fall apart or leak. It had a real nice Woods 42" mower, and a guy stopped by looking for something cheap to mow with and wanted something to fix up. As soon as he showed interest in it, I let it go. It was a huge project just waiting to happen, and I just had no interest in it.
Another 65 lo boy that I found on e-bay was down in the Pocono Mtns, had a Wagner front loader, fast hitch, and a flail mower. The price was right so I grabbed it. Another forum member wanted the front loader, so I stopped on the way home and dropped the loader off at his place. It came with a few other attachments and parts I didn't want or need, so I sold those off, and ended up with very little initial $$ in the tractor. That one tuned up and ran beautiful, didn't have a single leak anywhere, but was pretty chopped up, scarred, missing parts/pieces, etc. I went through it and turned it in to a new one. It turned out beautiful and ran just as good as it looked. I was taking pictures of it one day and my boys were playing around on it. One of them bumped the starter button while the other was climbing on one of the rear tires, and it threw him off. No harm, he didn't get hurt, but it ended up on the sale block the next week and ended up going to a guy in Georgia.
Sometimes, no matter what you think, the tractors just let you know whether or not you want them. I never thought I 'd get rid of a couple of them that I've had, but things change and it's just time to let them go. I've also sworn up and down that I'd never get another one that needed a complete restoration, just to fine one on Craigslist, or driving around out in the country and being in the right place at the right time.
I will say, if I were looking for a new one for myself, I'd start right here on the forum. Most of the tractors on here are well known not only by the owners but by the other forum members who have helped work on them over the years. There's a lot of fine equipment here, and always some nice ones for sale. There's also a TON of junk out on the internet, Craigslist, classifieds, etc., for sale! I laugh almost every time I look on Craigslist, at what some people are passing off as "perfect condition" "completely rebuilt" "runs like new".
Anyway, those are just my thoughts and words of ....., well, I guess I just better stop there.