Dell actually installs little if any of their own software on the server in a normal install, with the exception of the OpenManage software. OM is a great application, and you would be well served to install it. It is the easiest way to monitor all internal systems such as temps; fans; Raid controllers; and is also a valuable tool for for keep track of hardware changes; firmware, etc. It will come with the server, and is your best way to set up the OS.
Another thing you need to consider, is a 2nd processor. Your software says this is a requirement, if more than 10 WS are connected, and you indicate 10-15 concurrent. Personally, I think I would consult with their technical people a little more to thoroughly evaluate your infrastructure. You're really asking a lot from a single server; you're asking it to be your (sole) DC; your Exchange Server; your file server; your print server (presumably); anti-virus server; someone mentioned ISA server; and also a SQL server running what appears to be a pretty top-heavy application. A lot is hanging on a single server implementation here.
They mention MANY times about the use of thin client, such as Citrix and TermSrvr; and I can imagine why. Database apps like this can really wallow when they are utilized across a LAN. A lot of data is continuously pumped back and forth between the client and the server during queries and reporting, and that's the reason why so many accounting and management apps like this recommend a Terminal server (or Citrix) for the job. When this app runs on a TS, all the processing is done locally on the server- the client is just transmitting screen shots and keystrokes. It dramatically cuts down on network traffic since the data itself is not being transmitted back and forth. It also allows the use of some pretty wimpy workstations at the client side, since they aren't actually running the app or processing it locally. The server is doing this.
I think you're going to find your tech people will advise you to implement a scenario like this if you have 15 clients connected. And running terminal services sets up another whole scenario.
You cannot set up an SBS2003 server as a terminal server. Whether Citrix allows it or not, I don't know. But I can tell you from experience a TerSrv with 15 clients is going to be busy. I can't fathom it running on the SBS box even if it were allowed. This also means you would need to purchase SQL server as a stand-alone application, as the SBS install can not be put on another server. At that point, you would want to save the money (about 4-500) on the SBS Premium; downgrade that to standard; Purchase a 2nd server; purchase SQL server; and purchase Terminal Server (or Citrix) licensing.
Have they mentioned any of this to you?? It would be disastrous for you to spend the time and money on this setup, only to find it is completely inadequate for the main application you intend to run.