I would never buy a car that sat 18 years and expect it to be a turn key driver. It’s fine to buy it if you’re expecting it to be a project car. Seals, belts, hoses are potentially going to be dried out, brittle, or cracked. The car could have more than its fair share of rodents nests even stored inside. Even if they’re not nesting in the car, they do sometimes like the taste of wire sheathing and can chew up wiring harnesses pretty badly. If the car sat for 18 years, who’s to say that the tires aren’t even a few years older yet? Rubber that old just isn’t going to live long back on the road. If you’re serious about keeping and driving this car, a decent set of tires installed is probably going to run you $600 before you even get a set of plates on it. Depending on whether brake material is bonded or riveted, pads and shoe material could have fallen off their backing. There’s potential for a lot of parts to seize from sitting that many years too. Seized parts are really good at showing up nickel and dime style too. You can solve one problem and another shows up right behind it. All the 18 year old fluids and filters would have to get dealt with right away too. Engine oil, brake fluid, and coolant can all become corrosive over time.
I have a great example for you. I just acquired a Chevy Blazer last week that only sat a year and a half. I was told it had a bad fuel pump so I priced one out for $40 online. I immediately set a parts budget for a thousand to get it road worthy again and I’m expecting to go over budget. Along with the fuel pump, it got a new lock ring, ftp sensor, ignition module, coil, cap, rotor, plugs wires, steering gear adjustment, plus it’s still slated for a radiator, blend door module, and now I just found out today the entire rear axle is trashed. Not to mention a slew of other miscellaneous parts and materials.
One thing that could make this Mercedes a viable driver is if the the car has at least been periodically driven. If the current owner got the car out and drove it once a month, it’s could reduce some of that potential cost you’re facing if you do buy it. I’d still treat it like a project car nonetheless.