pfSense satisfies everything you need except the "daily bandwidth quota", AFAIK. You didn't specify the speed, which would determine what kind of hardware specs you can get away with; however, if 10Mb or under (standard ethernet speed), an old pentium-class PC with 128MB of RAM will do fine.
Keep in mind some things:
1. You said the ADSL modem has a wi-fi NIC on it. You will obviously need to figure out how to disable it and keep it that way. (Network security)
2. What is to keep anyone from accessing the ADSL modem directly, bypassing your whole setup? (Physical security)
3. If you're using wireless from the firewall to the other computers, you'll not only need to do a site survey, but make sure whatever wireless NIC you select is supported by pfSense; if 2.0, then this can be seen easily by seeing what FreeBSD 8.1 supports, which pfSense 2.0 is based on.
4. You didn't mention content filtering. Since this is your family, I could see you wanting this. Bear in mind that it will drastically increase the hardware requirements if you do this or any other major proxying or deep packet inspection with Snort or the like.
Good Luck!
Mike