Firewall advice

I would appreciate some advice regarding firewalls.

This week, I might be doing some network consulting for a small
business. Now, they have a small network on Windows 3.51. They are
planning on getting DSL for high speed internet connection, and
therefore will have a static IP.

My question is on the type of firewall to consider or suggest.

I have read some on configuration, but have never done one myself.
Though, by the time they get DSL, I am sure I can learn enough.

Should I consider, as some have said, a router with the firewall
protection built in, a ready made software package like www.netmax.com,
a light-medium script like TrinityOS, or should I struggle to make one
myself?

Their might be lesser expensive options than a Firebox, but I would
appreciate any suggestions. Also, not to cause a Linux/BSD war, but
which of these two OS options is generally considered more secure?
Netmax offers both as options.

Thanks for your help.

Mike Porter