I have both, and I prefer the SD. my 82 has served me quite well. I like the seats and the roominess of the SD more. also, it gets slightly better fuel mileage. simple way to tell if it's leather or not is to look for damage on the seat. if it's torn up, it's leather... also, the 82 has the simple climate control, the 85 has the kilma system, so a/c issues are more challenging. my 85D is in amazing condition, and I prefer the worn out interior of my SD... I have never had an SDL, so I can't help there, but from what I hear, it's more powerfull, faster, better fuel economy still, and more roomy in the back seat than the SD. sounds better all around. that head issue is a concern, but if it's in good shape now, some preventive maintenance will have it outlasting the 617.
YMMV

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!