ANy time you have accounting data on the local network and also want to be able to access it from a remote locatuion on the Internet you are going to need a faily complex solution. The simplest way to go is a VPN. Get a router that handles VPNs.
If it were me I'd have the data on a computer that acts as a file server. The server offers a lot of flexability. Likely the server is one of those server class machines with dual power supllies each fed by it's oen UPS and tha data is on a RAID. This way you remain on the air even if a common hardware failure happens. For backup you need a set of hard drives that you can rotate kepping one off-site at all times and the others inside a fire safe.
If the server lives on it's own LAN segment different from the other office computers then you can use firewall rules in the router to control access to the file server. Likely they will be on different segments, the office machnes on wireless and the server wired.
You'll be looking for an entry level router from Cisco not one of those cheap home wif routers For remote access you will need a stic IP address and good upstream speed on the Internet connection.
Some good ideas, but for a business that size I don't think I'd complicate things
that much.
Mac OS X Server can handle almost everything one would need to accomplish, including Internet routing, firewall, and VPN service, but it would require an investment in time to learn how to use it. I also prefer separate devices to keep from being reliant on a possible single point of failure.
I usually spec out Netopia routers for small businesses. They're relatively easy to set up and have good PPTP/IPSec VPN support and a good firewall, and they're very affordable. Use Airport Extremes or Airport Expresses to create a wireless network bridged to your wired network.
I'd also highly recommend a backup server of some sort. If you were running Leopard server and all the client machines were running Leopard as well, you could go for a networked implementation of Time Machine. I'd also suggest Retrospect despite the fact it's getting very long in the tooth (a major upgrade should be coming in a few months, hopefully). You can run Retrospect on the same machine as your main file server, but it would be even better to run it on another machine, again to protect against the problem of single point of failure.
For a small network like that, an Xserve is overkill. A dual-processor PowerMac G4 or PowerMac G5 would be fine, or perhaps a used Xserve G4.