Hello -
The gas engined MBs can be maintanence intensive. Perhaps your FIL told your MIL that the 10K expenditure was needed? Can;t think of her getting that info elsewhere. If it were me, I'd take the car, check it over for fluids, etc to make sure everything is as it should be and then drive it! That'll show any problems pretty quickly. No, replacing the PS pump will not cost $10k to replace. Right now we'd just be guessing to try and imagine what she might be referring to. Perhaps SP wires ($80-$120 parts) and SPs ($2 each), along with fuel ($22 parts) & air filters ( $25), cabin air filter, basically a 120k maintanence. Check brake pads ($40-$60 for parts to replace), adjust parking brake, water pump ($110-$180 parts), PS Pump ($250 parts)check that everything works - that kind of stuff. A waste of time IMHO.

Drive it and tell us how you did! You may have gotten incredibly lucky! But you should be prepared for spending a fair amount of money on maintanence each year. Get a workshop manual and maintanence schedule and find what your local hourly rate is for MB technicians - and you should come up with a close estimate of what it will run you each year. Surprises will always occur and you can add 20-30% to cover that stuff.

This is just a general suggestion of what you can suspect - people who own 500SELs probably have a better idea of what it will cost.

Good luck -