Only HTTP Proxy

Hi,

I want the BM "just" as Proxy Server for Authentication of the Users to the
Internet and control what they are allowed to surf is it possible, to
connect the BM just with one Interface and one IP Address (so just a
private, no public) to the Network?
Also, the BM won't be the default GW for the Clients this way, but the
Clients will have the BM as Proxy entered.
What do you think of something like that? Do I have to expect some problems
with that design?
Sincerley,
Tom