I kinda pieced my setup together. Just because I found a couple local suppliers. I would definitely go for at least 2 kegs. I found 4 on ebay for $100 with free shipping. I guess the amount of kegs all depends on how much you brew. I can normally keep all 4 full (and have some in bottles) with 10 gal batches every few weeks. Keep in mind sometimes you can find a sweeter deal on Craigslist. I've heard of some serious scores from Craigslist. I picked up my regulator online, but my CO2 tank I got from the local beverage vending supplier and hoses, taps, clamps etc from my local homebrew store. It's pretty easy to expand once you get the basic kit - like the ones at midwest or morebeer
To expand, put in a gas manifold and add picnic tap for each keg (if that's what you want to use). A more elaborate setup would be to get the through fridge shanks for each keg and a gas manifold/splitter or a tower with taps if you are making a mini kegerator. The possibilities are endless. It all depends on what you are working with.
Don't forget, you'll probably need an extra fridge too. Believe me - it's all worth it.