Replacing the seals is usually a good idea. Seal Kits are inexpensive. A good how-to on replacing seals is helpful.
The one tricky bit about o-rings is that certain types work best with certain kegs. This is especially true of the o-rings that sit between the dip tubes and the in/out ports. With the right o-rings, you really don't have to apply that much torque to the in/out post to get it to hold pressure. With the wrong type of o-ring you might have to really wrench it down, which can cause galling on the threads and shorten the life of your keg.
As for tools, not really. An adjustable wrench is fine. If you have a socket-wrench set with deep sockets, that works too. When you disassemble the thing, keep track of which part goes with which port. Makes reassembly easier.
Cleaning & Sanitizing is the same as any homebrewing equipment. Soaking the insides with PBW and StarSan is my normal approach. Soak the disassembled parts in the same stuff. These two episodes of Brew Strong (one, two) go in to the process a bit more. But there's nothing tricky about it.
There's not much else to keg maintenance. I like having a bleeder valve around just to make sure my kegs are holding pressure over time.