speedtouch 530 NAT versus linux pptp + nat

small setup - 5 PCs - users kinda demanding

I'm setting up a mail server (whitebox linux based). There is no hardware
left for a dedicated firewall (such as smoothwall).

I was originally tempted to use the speedtouch DLS modem (pptp that returns
a fixed ip) for NAT, and port 25, 443, 993 to the linux machine for smtp,
https and imaps access....but I'm concerned as to how 'robust' the
speedtouch is.

The only current alternative is to use the mail server as a firewall itself.

Are there any experiences or recommendations?

B.