I think any standard server will do for WSUS. we use the same server as SCCM for about 1000 with no issues. main thing is network bandwidth. put it on the fastest switch you have. HDD 1-2 TB, 32-64GB RAM will do.
here is my 1.5 pennies. you will only use it once a month. the other 29 days will be sitting there wasting server space, gas money and maintenance specially for 1000 nodes. down the line, you will use it for something else besides WSUS.
so, depending on your budget, go big now with more than what you need so the server does not become obsolete in couple of years. You can add SCCM or asset management software, file server, WDS, MDT, etc..
Best Sam ------------------------------------------------ Please mark as answered if this solves your issue