I'd look for something 85+HP 4wd or 100Hp + 2wd. Deere 40 series was brought up, very good choice. I would look for a tractor with the quad transmission. Only due to the power shifts (which I love) if they were abused can cost more to repair than the tractor is worth in that age.
I would look for the 4240/4440 already brought up, or 3150,3155, 4050, 4250,4450.
Keep in mind the 4x40 series Deeres are not 4wd but have a front wheel assist. The front wheels are hydraulic powered and do not work as well as a mechanical front wheel drive, which the 50 series are.
These would be my 1st choice.
2nd choice would be IH.
966, 986, 1066, 1086, 5088, 5288, 5488.
With these tractors the 66 series 4wd had 3 different front ends. The 1st ones had a regular axle and broke a lot, the other 2 axles had planetary front axle. These were much better, but don't turn as well as newer tractors. Hydraulics seem slower to me than a Deere of the same age. Also the IH use a torque amplifier for hi/low shifting. These were known to be a weak link when used for heavy tillage.
One tractor that has made a come back around here is the IH "two plus two" series, 3588, 3788. These tractors articulate to steer. I say make a come back because when it 1st came out farmers would put dual wheels all the way around the tractor and turn the engine up. Many broke due to not being made for this. This was from what I have read and my own experience working around the machines. Now I know several farmers with these machines and only put duals on the back. They have been able to buy them "cheap" and run them the same as they would a Deere 4450 or a IH 1086. One friend of mine picked a 3588 up about 6-8 years ago for around $9500.00. He has run it and last I asked him he was very happy with it. Another friend does custom work as a hobby and he has one that was his dads. He pulls a 5 btm plow and chops corn with a 3 row chopper.
The last year or so of the 2+2 IH did beef them up, these were 7000 series machines. They were suppose to be able to handle duals on all 4 corners of the tractor. I've never come across one of these, I don't think they made very many.
This shows the 2+2.
[ame=http://www.youtube.com/watch?v=PZwpWTJ3rpg]IH 2+2 tractors - YouTube[/ame]