I ran a home server with dyndns, just used it for family stuff. The issues I had weren't with bandwidth but with upload speed (important for your app) and mostl

I ran a home server with dyndns, just used it for family stuff. The issues I had weren't with bandwidth but with upload speed (important for your app) and mostly with uptime, between regular home pc outages, dyndns snafus, and occasional outages from the provider. in my experience, this would work for a "closed beta" but not for 5000 users. If you're willing to use python or (java) for the server-side stuff, take a look at google apps if you are price sensitive. You can host a pretty substantial site without paying anything.