Judging by signature line thats exactly what they've go now:
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Ginny in Denver-ish
78 300SD, 240K (mine)
79 300SD 175K (beater boy, his)
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Ginny - the engine vac shutoff can be "fixed" by plugging yellow vac lines at firewall with golf tees thus bypassing central locking system. Y'all can get heat bypassing the servo unit with T-valve and straight hose under the hood. AC at Denver am guessing y'all dont need. And 116 rust becomes truly fatal (makin the car unsafe) *only* where suspension components meet unibody: shock mounts, swaybar mounts for example rusting to smitherines.

And y'all oughta get decent scissors jack to replace the crappy jackpoint/socket MB hand crank jack anyhow. The MB jack's a tippy canoe notorious for falling over and dropping cars on any sorta incline. Personally I'd never crawl under an MB held up with factory jack alone, its that crummy.

My advice is absolutely keep the 175k mile backup 'mobile parts car' 300SD on the road as daily driver. Especially as yours at 240k miles might need cyl head valve train work in another couple years at about 290-320k. Then ye can bone the beater 116 for engine swap to git another 100k at least from the better car.

Dont let '"rust phobia" bother you unless it's effecting suspension/unibody integrity says me. Rocker panels, floorboards and whatnot just aint that load bearing to worry about.