at this point, you need to be talking to sales folks, not us...
The Small Business Server 2000 solution mentioned above will work but it is limited to 50 users...and you will paint yourself into a corner with it, imho. It looks and smells like nt/2000 but it is not, exactly. it will pretty much work out of the box and get all the jobs done on one server box. cheap and fairly easy to deploy. but you will not learn how to administer a 2000 network with it and when you need to expand it you will have to ask for way more money to convert to 'normal' 2000 network.
develop disaster plans. server failure. internet failure. for server hardware i would go ibm or dell, and do not cheap out. how many web hits expected? who will use the site? How big will the files on the file server get? will any applications be run from the server or all from the clients. exactly how will you backup each server (e.g. IIS-web server, exchange, sql server) how long a window will you have to accomplish the backups? who will need remote access?
i would get into a ms course on designing and implementing 2000 network. develop a spec swarming with numbers. fax it to sales folks (including ms) then research what they tell you. buy clients and servers with not opening the box during their lifetime (3-4 years)in mind. security basics: public servers in dmz's, windows updates automated for clients and servers, especially os, iis, sql. virus definitions automated.
budget for future software upgrades.
what will you do for your additional test network? you'll need one...what spares should you have on hand. how long can xxx be down? (internet, web server, email, file server). get overnight-type maintenance agreements and/or spares for critical equipment