All the external connections will be the same on all ball lock kegs. Most ball locks will have oval shaped lids which are interchangeable between different brands of kegs (even most pin lock). A few older style ball locks will have what's known as a "race track" lid on which the sides are straight instead of rounded. All race track lids are interchangeable between kegs of that type.
You will notice a difference in the base of the posts on various ball lock kegs. Most of the liquid posts will take a 7/8" deep hex socket although you will find some with a 9/16" hex base. The gas posts will also be found in both sizes. However, some gas posts will have a 12 pointed base instead of a six pointed base. Any standard 12 point deep socket of the right overall size will work on both posts.
The biggest difference between types of ball lock kegs comes in the threads on the fitting onto which you screw the posts. On some kegs, both posts have the same threads but on many the two are different sizes. The sizes of the fittings for gas or liquid posts are not uniform between different brands of kegs either. For this reason you can seldom convert pin lock kegs into ball locks (or vice versa). To see the difference check out the link called keg thread sizes on this site:
http://www.ipass.net/mpdixon/ That site also has all sorts of other great brewing info.
Inside the posts, you are a little luckier. There are only 2 or 3 different poppet sizes, so you don't need to keep a lot of different sizes around. You seldom have to replace these so there isn't much of a problem with that anyway.
Hope this helps a little.
Wayne
Bugeater Brewing Company