I'm not up on the value of used PCs, but it would cost a lot more than $450 to build using new components.
If you wish to connect 3 monitors to the single graphics card, I suggest that you web search for "eyefinity". Recent AMD cards support as many as 6 monitors off a single card, but monitors 3 through 6 have to use Display Port outputs. You'd either have to use a 3rd monitor that either had a DP input, or use an adapter. (Depending on which interface you wish to convert to, those adapters may not be cheap.)
Windows license? As a non-student, the cheapest for me would be a System Builder (generic OEM) license. The license for that doesn't permit the OS to be installed on a PC for your personal use, but somehow I believe that more than a few people violate that. (No enforcement.) You can probably do better through you college, but some of the student licenses sold in the past were upgrade licenses. That may not serve your needs.