I think a lot depends on her mindset. If she's a DIYer or you'll handle maintenance and repairs, no problem. If she's willing to establish a relationship with a good indy, no problem. If it's going to take an act of congress to keep her from taking it to Jiffy Lube for oil changes, then by all means she shouldn't get a 300D.

Does she drive in areas where Diesel fuel is available? It might be a tough sell if she has to go out of her way to refuel.

Sixto
91 300SE
87 300SDL
83 300SD