Your car should be in pretty good shape for that price. One was for sale here on the swap forum for less. The 124 head needs work at 200K+ miles. Mine went to 242K. New seals, valve job, lots of other parts $1300. Oil consumption went down from a quart in 400 miles to 1 in 2000. Do it earlier than in my car's case. I got a few scratches on my cylinder walls that I believe were caused by carbon buildup due to the amount of oil burned.
124 climate controls are known to be weak. I got all new A/C, control unit, lots of other parts $1300.
I don't know about the fuel injection. New injectors are $25 each and easy to change. The fuel distributor is expensive. I have no trouble with mine. My 560 has a fuel problem with a rogue sensor but I haven't repaired it yet so no data on cost.
My paint is very weak on the top surfaces, $1000 to $4000 to repaint.
So, find out what works on your prospective car and see what it costs to fix. To get a great Mercedes 300E, you pay a fair bit up front or you live with repairs over time. I think they are worth spending some money on.