First, if the guy is a respected mechanic, your questions will not been seen as pestering because he is most likely passionate about what he does.

Second, ask what maint records he has. Post back with what he says has been done and that will help.

Third, that price seems high even for areas with 240's as cult cars like Seattle or DC.

As for any 240 with about 150K miles you have number of routine times that need done now or soon (within 20K miles) if they are not in the records. Off the top of my head:

Front suspension bushings
Struts
Possibly ball joints and tie-rod ends
Rear suspension bushings (may still be good through 200K but can get expensive)
Center support bearing for the driveshaft
Output shaft bushing for the transmission
All hoses (heater, radiator, vac hoses)

There are also the issues related to rust. 1988 and later are so much better at fending it off but it is the hidden killer of 240's depending on your region.

The AC system is much better in post-1988 cars but it is still an R12 system, unless converted, in a 1990.

Every relay in the car is suspect but all can be repaired by re-soldering. If it is not a DL model you will have to deal with the window switches. Other electrical glitches are common and require basic troubleshooting skills which can be easily developed but are harder to master.

Normal tune-up items like the timing belt, cap, rotor, wires, and plugs might be due.

Finally, if the blower motor is not newer, it may not be long with this world. Replacing one of those is the best way to learn everything that goes on under the center console and dash of your new car.

All that being said, I am loving my 1990 240DL sedan. This forum makes no problem unsolvable except possibly rust and accident damage.

Mike