Not trying to hog the thread, but as far as tips for buying used....
1. Make sure the radiator does not leak, or the leak is small enough that it would be easy to have it properly welded and sealed. You should be able to pressure test it when you get it to be certain.
2. Make sure the fins are in decent shape. Some fins will get bent on a radiator, that is inevitable unless you have an appropriate cover for it. If too many of the fins are bent and mangled, it greatly reduces airflow and cooling capability of the radiator. The fins can usually be combed by a radiator shop to straighten them back out
3. Make sure the radiator doesnt have any JB Weld or stuff like that on it. Personally I hate JB Weld, but some people swear by it. In my personal experience, it should only be used as a band-aid until a permanent solution can be administered. Coolant seems to find its way through the JB Weld within a few days or weeks anyway.
4. I try to stay away from radiators that have had a stop-leak or silver seal used in them. If the person does not use it correctly, the stop-leak will plug up the openings for the tubes that carry coolant through the radiator.
5. Try to find a radiator that comes with the temperature sensor and fan. I think the temp sensors are the same in 1gen and 2gen rads, but I think the fans are different sizes and mount on the radiator differently. I just find it easier to buy it all together rather than piecing something together, especially for a simple system like the radiator.
6. And like I said before, try to find one that is for your generation of bike. I know there are some people on this forum with some wicked fabrication skills, but I can just imagine what my setup would look like if I tried putting a 1gen rad on my 2gen bike... I don't think it would be too pretty but maybe it's easier than I think. I like the idea of just hooking up a couple of hoses, clamping them down, tightening up a few bolts, and being ready to go.