Friends of mine are running a Mac mini as their server. They're a startup with four people and they're doing most of what you're doing (plus testing of their web apps), works like a charm.
If you're already planning for such a big group, the most important monetary factor are man hours, not costs of hardware (they're usually the cheapest in the whole budget). If you don't need
(1) redundancy of any sort,
(2) a lot of very fast harddrive storage,
(3) very much cpu power,
then a Mac mini would actually work for you. Judging from the list, I think a Mac mini fits your bill. If you need redundancy, an XServe or a similarly priced server from, say, HP should be given preference (companies like IBM and HP offer cheap servers with `normal' pc components, but they are `less redundant').
There is one additional thing to consider: if you get a `real' server, you want to add a small, sound proof rack to the budget. Servers are loud while the mini is virtually silent.
Overall, I think you'll be fine.