You'll need a CO2 bottle, the bigger the better, since they cost about the same to refill or exchange whether it's 5# or 20#.
You'll need an adjustable regulator with a gauge on the keg side, but not necessarily one on the tank side. The tank pressure doesn't change much from full to empty.
You'll need c-kegs. Ball-locks are more common, a little taller and narrower, so you can usually fit more into a space than pin-locks.
You'll need some hardware, like hoses, taps, quick-disconnects. I still use picnic taps on my kegs.
I once bought a kegerator from a friend and sometimes you see them on Craigslist, that came with a 5# CO2 bottle, regulator, sanke tap and an empty half barrel of Miller Lite along with the fridge. There was a lot of useful stuff for about $100.
Most all of my stuff came to me used and free. If you hang around long enough and let people know you are looking, stuff may come your way.