I don't like the idea of using the drain plug, either. I have visions of hitting a hard bump and having the hose get cut, scraped, or whatever and oil going everywhere. So, I was assuming I'd be tapping into the side of the oil pan.
On the thread size for the oil sending unit - beats me. It seems that that is the $64 question. I haven't really looked at it.
1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.