I have run AC on Leopard server as well. AFAIK it is essentially the same OS - the server version has some extras, but pretty much any app that can run on the client OS should run on the server version, too.
That said, the only reason to invest in the server version of the OS is to utilize those "extras" like directory services, etc. You didn't say if you needed any of that, so I'm not sure. If you don't you could simply run AC and/or the BIM server from a SL client workstation.