I would not do a valve job on a 250,000 mile engine. Its going to need bearings and rings soon enough. I would look for a complete engine on car-part and see if you can find a "euro specialist" in your area to swap it in for a reasonable price. A quick search shows there are plenty of 4.2 m116 engines out there for $1500 and less. That would also be a good time to do seals on the transmission if its been in there for a while.

I say this because this is not an AMG or otherwise special engine. I don't even bother with head gaskets on anything over 200,000 miles unless its a rare engine that needs to be rebuilt.

Another option is to see if you can just find a lower mile clean car with a price delta less than what you're looking at for repairs. Then let someone who is a more DIY get your car and keep it going.