Still have to say that hosting off-site makes much, much more sense in your case. $10-$20/mo versus $1,000 for the Mac mini Server, plus the extra RAM, a good UPS to ride you through any power outages, &c. &c.? Just to serve email for five users and do some light web hosting?
Are you at least planning on using the Mac mini Server for file sharing, calendar sharing, Time Machine Server, or any of the other Mac OS X Server services?