The seat belt should be fixed by the dealership or MB. Depending on where you live it might be the law (could be federal, I don't know) as for the headrest and the outside temp, who cares, not something I would worry about but that's just me.

I don't think 4k is a steal, but I don't think its too much either. Sounds like an avg price for an avg car. Of course if it really "has eyes" and is what you want it might be OK. How about records?