Hey, congrats!
I install and manage a lot of Dell servers. They are cheap and use good components. They are VERY commonly found as Small Business Servers. Even the lowest-end box, an SC430, would be OK for a ten-person office. Get at least 1GB of RAM, although SBS will run in 512MB. All Dell servers use error-correcting memory, so memory errors are pretty much non-existent.
If you are technically inclined, you can install a second SATA drive yourself and have a RAID 1 array, so if one drive fails, the server keeps running. Be sure to get a floppy drive with the server.
There is NO decision to be made about software. SBS 2003 is the obvious choice for most small businesses that don't have an IT department. Dell has had deals with basic SC420s or 430s for $600 including SBS 2003 Standard Edition, but that's not a normal occurance. The Premium Edition has some definite reasons for existence, especially if you need high-end accounting systems or CRM software that needs a SQL database or if you want to host public web sites. And the included ISA 2004 firewall is excellent if you want to monitor or control employee Internet usage. But the Standard Edition works fine for lots of companies.
You might consider buying SBS in the Retail version, instead of taking Dell's OEM install. It's less than $500 for the Standard Edition and gives you the option to move it to another Server if you later decide you want to move SBS to another box because your business or your server usage is growing. SBS itself will work fine for up to 75 users.
Be sure to plan on where to put your backups. The simplest and cheapest way is to have at least two external USB drives. Keep one attached and keep the second one offsite. Rotate them. If you do that, SBS will pretty much take care of the backups for you.
btw....once you get your server, you can easily set it so that twenty-character passwords are required of everyone....an easy way to avoid those doe99 passwords and get people using pass phrases.
I have some initial configuration hints
in this AnandTech Forum post.
If you want to host your own email system (part of SBS and HIGHLY recommended...it makes email so much easier to manage and conrol), you'll want a business-class DSL connection. The difference is that they don't block port 25 (email) and 80 (web). It doesn't have to be a fast connection, but you don't want those blocked ports that home connections have.