The PO either got old or got tired of replacing parts without seeing progress. That is how I got a 600. One 4 digit bill too many for the guy. The car now sits waiting for me to pay a 5 digit repair bill, so at least it's not costing me money to maintain it.
Replacement injectors are pricey. I'd be wary of replacing just because they look shabby. The wire harness, that's different. It will be brittle and frayed. Addressing that never hurts (well, except the wallet).
Once it runs nicely the next step is rides nicely. A 300sel has a rather unique air suspension system that should give you endless hours of comfort and joy. Not to mention more work, once something needs repair. When it works, even moderately well, it is in a class of its own.
-CTH