Speaking from experience, use a network appliance to do whatever work you can. By this I mean that you should offload the work of DHCP and DNS to a firewall/router instead of trying to configure it on the server. It's a lot easier to cycle the power on a router than it is to trouble shoot a Windows server that's not issuing IP addresses properly [Linked Image from sundance-communications.com]

Also, when you order the DSL for the customer, be sure to specify that you want an ethernet DSL modem. Some ISPs will send a PCI or USB DSL modem out by default. You do NOT want one of those. Getting it working consistently is a nightmare. With an ethernet modem, you just plug it into a firewall/router and walk away.

For remote connecting, I'd recommend running logmein.com on one of the workstations in the office. It's secure and works through a firewall. I use it at a lot of my client's offices.