Pin-lock VS Ball-lock pros/cons
Pin-Lock Pros:
Cheaper, can't mix up in / out QDs, shorter so they don't need as much "head-room"
Cons:
Harder to share kegs with someone else as most people have ball-lock. Not as easy to find, generally. No new ones being made. Can't run CO2 through the out post (to help carb faster, etc.) wider so you can't fit quite as many in the same space.
Ball-lock Pros/cons:
PRO:
Easy to find, most people have ball-lock connectors, skinnier than pin-lock so you fit more into a given space, you can run CO2 through the liquid out post to help force carb faster.
CON:
Easy to mix up the in / out posts / connectors, taller so you need more head room, more expensive to buy
Just about any keg you buy used will need to have new rings put on it. You should thoroughly test it to see if it holds pressure. If it leaks around the lid, that's not usually a deal breaker. What I would do is take a 5# CO2 tank, put some water in a keg you're planning to buy (if you buy it locally) and pressurize it and then turn it upside down. As long as water doesn't pour out, you should be good.

Also, some of these used kegs are not cleaned before they are sent out; they just have whatever residual syrup is in them dumped and they are then sent out. You should definitely clean and sanitize any kegs you buy before using.
The advice by Will Shakes Beer is good... buy your kegs and test-fit into whatever you're looking to buy for temp control. As far as regulators go, Tap-Rite is hard to beat. They are the "standard" regulator, but there are probably some higher-end ones that may have a few bells and whistles. If you are wanting to possibly carb up multiple kegs at different pressures, you should consider a primary and secondary regulator so you can adjust pressure. Always put your higher pressure on the regulator closest to the tank, and your lower-pressure on the secondary.
As for tanks, there's a guy here on HBT that sells refurbished tanks that you can either fill or exchange for a full one. His prices, including shipping, are excellent. A lot cheaper than buying a new one.
