Ah! Now I understand.
You want the server to be intermediate between the Clients and the outer Internet.
What you want is what an Ultrapeer does for its leaves. If you want to exclude search results from inside the lan, you can do so with the sae settings which could exclude searches from outside, but except for pushes, downloads are normally performed by the individual clients themselves.
Every client will only keep a connection to the single Ultrapeer (and most expectedly will have a connection overhead of less than 500B/s).
Phex is at
http://phex.kouk.de and the sources at
http://sf.net/projects/phex in the CVS.