Replaced the valve seats??? That doesn't sound right. How about the valve guides and seals. Verify what was actually done. If the head was recently refreshed that is good!
The Merc 2.6L and 3.0L in line sixes of that era (called M103) have timing chains, which are generally trouble free if the oil is changed within the manufacturer's recommendations. These engines are torque shy, but love to rev, and are smooth as silk. Check the engine for oil leaks particularly at the front and right rear. Also check for any coolant seepage at the head/block interface, and check that the climate control system operates properly.
Check that the engine has a steady idle speed in drive and neutral. These engines tend to have high emissions, so if emission tests are required in your area inspect the latest emission test report and any history you can come up with.
The 190E (W201 chassis)is essentially a 7/8 scale version of the 300E (W124 chassis). IMO it is probably the best looking and proportioned small sedan ever buiit. It has about the same front seat room as the W124, but a smaller back seat and trunk; 190s are basically good cars, but like most Mercs they are complicated, and problems with systems like the KE fuel injection system and climate control can be expensive and exasperating.
For the mileage the asking price is reasonable if the car is a "cream puff".
'88 190E 2.6, five-speed manual, 73K miles