This is a rare car. The steal in the 1991 350 SD is different than all the rest of the lineup that year being lighter gauge but higher strength to make the 350SD lighter and it was. If you have an original engine, it's a time bomb. It will happen, the bent rod issue. I had three bent rods in my "original" engine. Top speed speedometer indicated is 112 MPH with the bent rods and the engine needing a lot of oil all the time. I put a new long block in and turbo with the updated connecting rods and the oil consumption is normal now. Also the top speed is still 112 MPH. I was able to adjust the turbo boost pressure to 1.0 bar and this helped acceleration.
I bought one of these in the spring with a rebuilt head and rebuilt transmission. I really liked the car and own a 300sd, but I could not get it to run as well as I would have liked and I've read a lot about them and took as good a car of it as I could but it threw a rod I think, going highway speed it made a huge smoke cloud I pulled over and tried to stop it but it kept running and revving and then stopped. Basically totaled. Take my warning, I had read bad reviews of the engine but didn't pay attention.
My 350SD has been a daily driver for me for 9 years. This is one of the best driving sedans mercedes ever built. The car really handles a curve and is a great touring sedan on the hwy. I like the diesel, it's no dog with the the turbo and I just typically drive a little slower (no tickets). Now this is very inportant if you own this car; The turbo charger on this engine gets its lubrication from the engine oil, the turbo charger operates at such an high RPM that within a very short time "60K to 80K miles of engine operation" the seals in the turbo can go and will start blowing engine oil into the manifold. This will lead to engine loss. If you're losing eng oil replace the turbo immediately.
I hear a lot of people complaining about this car. Personally I love the thing, it's great on gas, and is very smooth. But, it is also supposedly the worst diesel ever made by Mercedes because of the weaker connecting rods in the engine. Apparently they bend and the car starts to burn oil. I would rev the car up very slowly and watch for plumes of smoke coming out of the exhaust at certain rpm's. If they are there, that would mean that the rods are bent and the engine is going to go real soon. Buyer beware.
Only had the the car for a year but virtually no problems mechanically speaking. Gets amazing mileage for a brick on wheels (low to mid 30's normally), driving in snow and ice with so much torque in the rear end takes some getting used to but its for your own good. Honestly when they say they don't build them like they used to this is the perfect example.