To properly address this thread, you'll need to provide me with a little more information. Like what need does this business have for a PC?
Accounting and Book keeping? Inventory? POS system? Internet? Email? Multimedia Video Displays? PVR capability? Just a few needs...
Then a budget. Duh... lol j/k
If there is one thing that I always recommend to people are going to buy or have me build them a new PC for a business. I suggest you invest a good portion of your budget into protecting your data.
If your aren't using a server to store your data and you're going to be storing all of your data on your PC's HDD's I would highly recommend that you consider setting up a RAID 5 configuration.
I'd recommend Seagate 200gig SATA 7200rpm with NCQ
RAID 5 stripes data & parity information across 3 or more drives.
This is a very safe way to ensure that you secure your data from HDD failure. It's not the fastest configuration, but it's very safe. IF one hard drive fails, just swap it out with a new HDD.