The number 1 thing is rust - if there is any rust on these cars it tends to spread because of the way MB undercoated everything. You cant see it until its too late. So - around the jack points - the holes in the rocker panel under the doors - check real good - if those are solid you have a solid car.

Most of the accessories are run on vacuum not electricity. For example - when you lock the drivers door - all three remaining door locks should slowly go down. When you unlock they should slowly go up. If they dont - it is a Serious pain in the A$$ to work on. If your not capable of doing the work yourself it gets expensive.

Other than that - check out the transmission shifts OK - 330k miles really isn't out of line for one of these cars - a well cared for engine can go 3x that without much work at all.