ADSL modem & ip addresses

[Ian Moore]

Hi,
I'm organising an ADSL connection and I'm a bit confused about our options.

We need to provide web, ssh and mail access to our network for users from home 
across the Internet with an ADSL connection.
I figure the best way to do this is to setup a new machine to act as a 
firewall and run a web server & sendmail on this box. (or I have seen 
something about using socket to divert these services to our existing server 
which has a private address).
The firewall would have a NIC with a private IP address to connect to the rest 
of our network.

What's the best way then to connect it to the ADSL line?
Do we have a second NIC in the firewall machine with a real IP address 
connected to an ADSL modem and use ppp -natd on that interface? Does that 
mean we'd need 2 static IP addresses - one for the firewall & one for the 
modem? (We really don't want to pay for 2 addresses)

Or can we use a USB connection instead - are there FBSD drivers for ADSL 
modems? I can't see any in the supported hardware list.

Or do we use a combined modem/router device to do the nat & firewalling and 
have it redirect mail, web & ssh access to our main server? (is that possible 
or do such devices not allow access into the network from the 'net?)

Cheers,
Ian