Guys, please feel free to add or comment further, you thoughts are most inspiring and helpful.
Have a look at Freesco http://www.freesco.org - nat, firewall, router plus some other things - runs from a floppy on a 386 upwards with 16megs ram (or less with a h/d for swap). May or may not do what you require but is definitely a v. cheap alternative. No USB support though.
HTH Syd