The suggestion below to search for "1u server" is probably good to get a feel for what the overall used market is like and what relative availability is. It helps to keep up a little with the product line of whoever moves a lot of equipment in your area (Dell around here) so you'll know what the popular/good models are to watch out for when they hit the used market. It also helps to know what options are available and/or common for the model you're looking at so you know if you're seeing something that's specced up or bare-bones or in between.

Server manufacturers usually have someplace on their websites for hardware or technical guides or spec-sheets. They're often available for 10+ years after they quit selling a model, so you can research a system before you buy a truckload of them.