you should know that mac os is loosely based on unix architecutre, effectively making it a unix machine when it comes to its suitability for use as a web server. imo getting a mac for this sole purpose is not worth it, as you could get a better x86-based pc for less money and use it with unix/linux for this same purpose. almost any flavor unix/linux comes packed with tools to make server configuration easier. if you must have a mac, a g3 will do just fine for your purported purpose. however, you mentioned that you wish to use this machine as a server for your lan. if your other machines are using an ms-based os, the simplest way to network them is to use an ms-based system on the server node as well.
my suggestion is to forget the mac, get a pc and use windows 2003 server on it. it is very stable, extremely easy to set up as a web-server, and given a good anti-virus software (i reccomend mc-affee or norton enterprise) it is as resistant to viruses as anything. it is by far the easiest to set up, configure and use. people scoff at windows for various reasons, but fact is that of all the available platforms, windows has the highest degree of usability and overall integration. before all the people on this forum write angry responses to me, consider i use two linux servers and a unix server daily. i have quite a bit of experience with all four major platforms: ms, mac os, linux and unix. plus, this is just one man's opinion. hope you find this useful.