Im looking at buying a 74 450se that has sat in place since 1980.. The cars paint is effed, but I kinda prefer that, less to worry about, I figure all the rubber has gone bad by now.. but the mechanical history over its 117,000 miles has been superb so I figure after fluid flushing and changes i might have a running vehicle, and potentially a well running vehicle. The interior is nice and the body is in good shape and not rusted out and minus a small dent in the passenger door it overall looks workable.. The old quote is that there is nothing more expensive than a cheap Mercedes, and I believe this, but I'm hoping this wont be too much for me. I'm used to an OM617, and Ive never worked on a M117 before but I am quite determined. Is that rear suspension on the w116 as big of a monster as it seems as well? If I decide to buy it, what are the Must do's and must not do's?

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A junkyard is not for junk

Open the doors and watch closely if they drop a bit.
Get a big screwdriver and poke arond the firewall where the kickdown switch is ( if it's auto) even if it's manual poke hard, if it goes thru ?????.
Poke the rocker panels and the bottom of the doors hard as well, if the screwdriver goes thru ???
Stick your head behind each front front wheel and poke your screwdriver at the panel that stops crap getting into the door pillar area, if it goes thru ???
Poke hard at the big box section where all the front suspension mounts, if it goes thru ????
Been there, done that, seen the movie and got the very expensive T shirt.

I'd want to know why is sat since it was 6 years old and the mileage is so high.

This would be a sub $ 500 car due to it needing:

Fuel tank removal for cleaning or replacement + fuel sender is going to be badly corroded.

Replacement of fuel lines, they are probably plugged solid.

Replacement of fuel injectors / pressure regulator / fuel pump / fuel filter

Replacement of every component that brake fluid touches. Master cylinder all the way through to the calipers.

All cooling system hoses, water pump.

The head gasket will likely be failing due to corrosion.

At least one intake and exhaust engine valve will be open to air, I'd expect those cylinders to be rusty. Yes, if the engine is stuck you could break it free, but the rust will soon wear out piston rings / pistons.

The above is just for starters, you still have to fix why the car was parked in the first place and take care of other things that deteriorated on their own. Don't let the 117,000 miles fool you, if the car had 3,000 miles you would be doing all of the above.

I'd say get stuck in, and take it slow. Doesn't sound too different to my w111, that was sat for 27 years. All rubber had to be replaced, not too difficult, just time consuming. Fuel system re built, all fluids, brakes, ignition, cooling, all switches rebuilt, injection system rebuilt, etc etc etc. None of it was too hard, just lots of reading and thinking, lots of time and a fair bit of cash. Some parts are ridiculously expensive, but in 90% of cases I've found a way to do things cheaper..mainly through patience and research. Very rewarding resurrecting an old car like that. Unless you're minted however I think the only way to do it is to do it yourself. Good luck.

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1971 W111 280se rhd coupe (The Duchess)
1959 Bristol 406 (Monty)
1970 Volvo Amazon 131 (Lola)
Lives in Cornwall, UK.

Amazon69.
All of what you say. I almost gave up on mine after a year. I know at best if I sell it in the future I'll be lucky to even get near break-even on the money side but the satisfaction factor is good.
Mine is 99% there and the first time it goes back on the road legally will be big daft grin day.

Most hobbys cost money and have no financial return so it's hard to put a price on what it's worth. And then there's the element of getting in so deep financially one loses interest before it's done. If it's something you can put minimal cost into to get results that's great but you can't do much without spending money.

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Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT

Halfway through my rebuild, floor up, I sat down and added up what I'd spent. At that point it was stop or go for it.
I decided to carry on but at a slower pace.
I think at the end it will be worth it.

The 116 isn't a collector's car yet, and may never really be one. Sedans just don't bring the big bucks. The 74 you're looking at was D-Jet so you don't dare even want to think about trying to start it without replacing every rubber line under the hood, because it's a 30 PSI fuel system and it will cause a fire when those hoses that've sat since Carter was President get pressurized.

If you want to buy it as a project car, it'll be a fun project, but assume everything on it is shot and you're buying a shell. $300 is probably its scrap value so that's what it's worth.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

You haven't mentioned what you want to use the car for? If your end game is to drive I would let it go. Project cars don't always have to be about money but for me it is a factor. Plus this car has just been sitting too long...and outside.

As mentioned by another here, those sedans are not worth much so your time would be better spent on a more desirable model. Besides the obvious reasons for this comment, another is that projects often times don't get finished and this car would be very hard to "off" in the middle of its resurrection.

If you want a car to drive, get one running. Someone I know just picked up a 1991 300 SE for $2100. One owner, 135k miles, original paint, champagne color, leather, only big issue with it is the AC doesn't work. Garaged, so the paint and interior is an 8/10. My point is there are better cars out there for less than it would cost just to get this car to run and stop.