You don't necessarily need a top-end server. It depends on a number of things.
Microsoft's minimum requirements (this is available on the Microsoft website) for Win2K server are:
Pentium 133MHz or higher
128MB RAM
2GB HD
While Win2K willactually work with this configuration, you wouldn't be happy with its performance.
What are the requirements of the accounting application? How many of your users are likely to need to use this application concurrently? Will the server be used to perform any other functions? Can the accounting application make use of an SMP (multi-processor) server? How you should configure your server will depend a lot on the answers to these questions.
Just as a baseline, I would suggest the followingminimum to start (based on currently available components) (actual options available may also depend on whether you buy from a vendor, or build it yourself):
Pentium III 1.4GHz
512MB RAM
Minimum 4GB system/boot partition for Win2K server installation
As far as where to look for a vendor, I always recommend Dell (www.dell.com). If you do go with Dell, and talk to one of their customer service reps (vs. configuring and buying a system online), if you have the answers to the questions I asked above, it will better help the rep configure a usable system without having to buy more than you need.
Hope this gets you started.