That's a tuff question. I think you should consider how long you expect to keep the car. You already have $2100.00 in a car that's probably worth about that much. However, if you plan on keeping it, it's worth pursuing.
If it were me, I'd set a limit as to how much more I'd be willing to "invest" to get it running. I say invest because, once running, you should get a lot of pleasure driving it. Set a limit because, if it just won't run right, you're going to take a loss.
Who knows, it could be something minor. Unfortunately, you are off to a bad start. IMHO, any 123 Benz that needs a motor or tranny isn't worth more than $750.00, for the reason you are experiencing. A used motor is a pig in a poke, unless you heard it run. A compression test is a great idea... before you buy it.
The best advise I can give, read this forum. you'll learn a lot!
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