The product for this is M$ Windows Small Business Server 2003, you'll need the premium version if you need ISA. This all runs on one standalone server. All your workstations need to be Win 2k or Win XP Pro (home won't work). Max users 75.
It is a steep leaning curve to setup, if all you've had before is a windows peer to peer network, but a very flexible product when you get it right.
I wouldn't try and host a business website. Your better off paying for hosting so the webserver can push that data out. Just allow access to the server for the company users to connect and get their email etc. Outlook can be configured to allow web access, and other software also enables connectivity.
There are some good websites on SBS including
www.smallbizserver.net.
Also check out the book: Windows Small Business Server 2003, Best Practices by Harry Brelsford.
As for the server, doesn't need to be phenominally powerful. I've got a Dell PE2800 with twin Dual cored 2.8 Xeons and 4GB RAM. You need to maximise RAM, but not the CPUs, one Xeon only would have done the job for our 20 users. Indeed I had to disable hyperthreading on the Xeons as ISA only support 4 CPUs not the 8 detected by hyperthreading. Key thing to think of with the server is data backup and quantities of storage, you also need to have a corporate level antivirus - not all integrate with SBS - I use Panda.