the 831 for the pricing sounds about right. you may find it cheaper with more research but that's not bad.
the 831 will provide IDS for intrusion detection, firewall services for NAT, stateful inspection and port blocking, easy VPN for up to 5 users, IPSec and QoS as well as many other features.
please see the following link for more info on the 830 series router:
http://www.cisco.com/en/US/products/hw/routers/ps380/products_data_sheet09186a008010e5c5.html
as far as your thought process, it is not too far off. the biggest issue you may have is the 100MB port that will connect to your 'dmz' where your servers reside; is it enough to handle the anticipated aggregated bandwidth for the servers you mentioned.
(it should be for the most part but not knowing your true bandwidth requirements, we cannot say for sure at this point; if these servers are mostly access via the net, then this should be fine)
one other option is researching on the net for other routers more powerful such as the 2600/2800 series or the likes.
(there are alot of good routers available at ebay and other sources that are more powerful than a 830 series for the same or close to the same price; it will take a bit of looking around but they can be found if you so desire)
if this is absolutely mission critical, which it seems it would be since you run your business off of it, you may want to settle for the piece of mind of having the device come from the manufacturer rather than an unknown on the net.
(plus you can get maintenance from cisco, etc.)
