McWatters my man:
http://www.bmw4life.com/mTechSkirts.htm
As for the lower door trims (actual name), they should be in group 41 or 51 in ETK.
You'd be impressed -- the only areas that really rust on the E34 are around those trims, the lower rear part of the front fenders, and sometimes around the rear quarter panel and trunk. These cars have excellent rust protection underneath, and it takes a lot to kill one.
There are a few ways to approach the current rust. I'd get some POR-15 rust convertor for the non visible areas and paint them with it. Visible areas should be sanded down, body mudded (accordingly/if necessary), and then primed and painted.
More on skirts and such/where to find them in my article above... The rust issues should be dealt with before installing skirts, anyway.
best, whit