That Mercedes S320 CDi, with that crap G-Tronic transmission, has massive turbo lag. If you put a bigger turbo you just increase that (and that's bad). It's a heavy car, a diesel with automatic transmission; not really meant for track days. You can't polish a turd. And by that I mean: leave it as is, drive it, enjoy it.
And BTW, it's a diesel... How do the "bigger spark plugs" increase performance?!