My company is a medium sized credit union on California, and we brought BB on board about a little over a year ago. We have 18 units, and 2 mobile phones deployed thru our staff.
The Enterprise Server (BES) is VERY advantageous to have, but it helps to have an IT guy who can do all your installation and upgrades. If you have an person in charge of IT (and can afford the one time expense) I think you'd find that to be a great thing to have, especially if you do a large portion of your business via e-mail, or need to be in constant contact.
As far as the devices go, they typically run around $299USD with various and sundry discounts. We are moving from one carrier to another, and upgrading from the 7280 comparable unit to the 7290 unit, at $75USD per unit. We already have the BES 4.0 and the new firmware is preinstalled on the units. For six units, its likely you'd have to pay close to retail price, but shop around for the best deal you can get.
You're geographic area will determine what carrier is best for you...T-Mo, Cingular, and Verizon all have different programs for small business, but if their coverage is crappy in your area, it cheap hardware doesn't benefit you much (we found this out the hard way)
Good luck to you in your search...our lives have not been the same since we acquired our devices. There two timelines...BBB and ABB - Before and After Blackberry.