Since you are specifically asking about server setup, I’ll assume that you are either talking about a dedicated server that you’ll be leasing from a hosting provider, or a box that you will be setting up from scratch at your home or office.
Unfortunately, there are just too many variables to give a patent answer to that question. But here is a BP thread that might shed some light:
https://buddypress.org/forums/topic/buddypress-and-shared-hosting
For everyone else wondering what the bare minimum setup they need in order to have a healthy working relationship with WPMU + BP, I always provide this caveat: don’t expect to get great, or even decent, performance from a a shared hosting account. It just won’t happen. Of course there are some that claim to have several hundred active users working without issue on a shared account. That could be the case. However, in my opinion, a shared hosting setup is out of the question for a really robust, business-oriented site.
There are many, many threads over at the WPMU forums on hosting providers, server requirements, etcetera. That is probably the best place to seek advice on such matters.
