Where do I find the minimum hardware requirements for the WSUS server for a network with 300 client computers? I looked on Microsoft but I don't think Im looking in the right spot.

Pretty low, as WSUS has been around for ages.
Server 2000, 1 GB RAM, 2Ghz CPU.
The real killer is the amount of HDD you're going to need for the dowloaded updates.
I run a couple 100 GB and I remove old updates regularly.