It's nice to have at least two, but you can always add on.
I wouldn't bother with a set up, just put one together yourself. You'll need a C02 tank - the bigger the better. Only go with a 5 lb one if you can't keep it outside of the fridge. 10 lb is a nice size. Check local places to see if you can get on a tank exchange. You trade your tank if for a full one that way and don't need to worry about testing.
Next thing is what kind of fridge you have. If it's an upright, you want to go with shanks. If it's a chest or mini-type fridge, you may want a tower instead. Check micromatic.com and beveragefactory.com for the best prices. You can go with cobra taps too, but only do that if you're on a tight budget. You'll need a primary regulator for your tank too.
Spend the extra money on front sealing faucets - you'll be glad you did.
You'll need corney kegs. You can get those from http://www.homebrewing.org/
. Get the ball locks unless your fridge is too short for them.
Last, you'll need all the connectors and tubes. I think Beveragefactory. com has a nice map of all the stuff involved. For serving, most of us go with 3/16 ID. For the gas, use 5/16th.
That's pretty much it.
Here is info on balancing