I have a 99 ML 320 with about 170K miles on it. I am the original owner. While replaceing the battery over the weekend, I discovered a significant amount of rust in the battery storage area. Is this common?

I wire brushed the rust. Then primed and painted with Rustoleum products. Also put a battery mat under the new battery. Anything else you can suggest to prevent it from recurring?

Typically the rust/corrosion is from leaking batteries.
I think you did the right thing in priming it. It's a lot better than to have a body shop cut the tray out and welding in a new one.
Makes sure your drain tube is installed on the battery. Battery water level is always properly filled, not too low or too high. And just keep and eye on the battery, check it at every oil change (3k not 10k)

There is no vent tube on the battery. It is a Carquest low maintenance model. Essentially no maintenance is expected, but it does have screw on caps to permit addition of water if necessary.



Should there be a vent tube?

There should be, but it's for gas that's created in the battery.
If you keep the fluid level correct you won't have a problem.