If you want to offer public wifi I would suggest doing so through your existing router and getting a seperate router/firewall for your home LAN.
Domestic and SOHO routers and wireless access points rarely have the features you would need to do what you propose safely.
You could also splash out on a more expensive router that offers wireless isolation, and multiple SSID's
Draytek routers are good.
What sort of internet connection do you have ADSL, Cable, or BT Infinity (fibre to the cab)?