You didn't say what kind of tractor you were wanting to get. I assume your looking at some type of used farm tractor probaly with a loader.
A real good place to find out information is at the Antique Tractor Site or Yesterdays Tractors ( same site ).
www.ytmag.com
There is all kinds of info on the question you are asking, plus a lot more than you would care to know about tractors such as production numbers, history on different models, things of that nature.
If you are looking at a farm type tractor stick with IH, J Deere or Ford these are the most popular and most parts are readily available for these tractors.
If you do find a decent tractor and the engine is a little rough and smokey I'd use this as a barganing tool on the price. Engine parts are easy to come by compared to trannys and body parts.
Take some tools with you to pull a spark plug or two, check fluid levels if it doesn't have dipsticks, battery/ charging system meter. Your the buyer you have to live with the machine after the sale, if the seller has nothing to hide he won't mind.
When it comes to used machines it's buyer bewary, I don't hold stock in anything the seller says unless I know the person. What one person considers to be good and what I consider to be good is just a matter of opinion.