Only the '95 E300 and 92-95 500E/E500 have 90 liter tanks. I don't know if gas and Diesel use the same tank. The larger tank can be retrofitted to any 124 sedan. It's up to you to swap over the trunk liner parts or trim/hack the existing liner parts.

A 2.5 or E300 has to be in tiptop shape to do 0-60 in 13 seconds considering they're closing on or at 20 years on the road.

I don't know the context of David Wilson's problematic characterization. I wouldn't say the '87 is the more or less problematic than other W124 Diesels. The potential for the early head casting to crack is well documented but you'll be bucks ahead to get into a well maintained '87 than a neglected '90-93 or '95. You break a glow plug in a '95 and the head likely has to come off to be fixed.

Do you plan to service the car yourself? They can be expensive to maintain otherwise.

Sixto
87 300D^2