A 300SDL is enough of an improvement over a 300SD to make it worth a look at any available to you. For some reason, my SDL doesn't pull as hard off the line as either of the SDs I had, but once it gets rolling it keeps pulling until you're ahead of traffic. Both mileage and power are better than what you'll get from an SD in similar condition. It's also quieter. I don't know if it's a quieter engine or the doing of encapsulation panels.

The original cylinder head of an 86-87 SDL or 87 D/TD is susceptible to cracking. A new replacement head will cost you $2500/DIY to $4000/shop. I don't have stats on how many develop cracks.

Sixto
95 S420
87 300SDL