is a good thread on kegerator builds that have been done by other members to give you ideas.
thread has different freezers and how many kegs they hold (method there is good- ~9' diameter and 28' per keg).
is a kegging 'kit' from midwest that is a pretty good starter to model after.
For your CO2, it is cheaper per pound to fill a larger tank (IE 5# fill is about $10 where I go, and a 20# is like $14). Unless you plan on keeping it in the fridge, where size constraints are an issue, it wouldn't hurt to have a big one (also less trips to refill). 5# should still last you 6+ kegs (carbing serving etc).
I recommend spending a little extra on a dual gauge regulator, unless you don't plan on having more then one or two on tap at a time (this way can do different pressures for serving/carbing).
'Y' splitters (or distribution block) with check valves and on/off switches if you need more CO2 connections (how many kegs you are going to have hooked up).
For each keg you are planning to serve, you will need a gas in connect and a gas out connect (ball or pinlock, depending on keg. The ball locks are more common, also what is shown on midwest link above).
Typically barbs for gas are 1/4 ID tubing, to any desired length. The serving lines are 3/16; it varies per your set up, but 10ft is common length for most set ups (see draft system line balancing
and/or Beer Line and Length Pressure Calculator
Other then that, picnic taps or perlick faucets
(THE faucet to go for in my book), fridge, tower if you're doing taps...
I think I covered most of it?