Special captive portal

Hi,

Suppose I have a wireless network, with different AP linked to a
router-modem (same device). I decided that, internet access should be
granted to only clients with logins. I don't want them to exchange
logins (so 1 MAC to 1 Logins).

I thought about installing m0n0wall as a captive portal and I read
about Local User Management which apparently looks like exactly what I
need.

Besides, could you suggest an implementation of this system, 1 MAC <=>
1 account, without asking the client for his mac address beforehand.
How to get the MAC address of the first NIC to log in with a fresh
made account? How to add it later as a rule in logging process in
addition to valid username and password.

I would be pleased with any of your suggestions and advices

Thanks