[Perfectlap]

I don't think that's a good trade. The big question is will this be your daily driver or your weekend car?

For one thing, not all 75k miles are created equal. Are those year round miles, meaning the car was driven on winter roads? If so the car's suspension has had a work out and typically will not have much left in the tank. If its still on the original clutch/RMS that doesn't have a whole left either and you'll be looking at $4K once you toss in the IMS/flywheel. New rear axle boots while your in there.
Known Boxster weak spots have to be factored in like the water pump, coolant tank, AOS and high mileage stuff like the rear tires (possibly all four), rotors and pads. All this stuff ads up.

I'd hold out for a lower mileage S model, preferably one where the seller can tell you straight away if the IMS was done because the previous owner was an enthusiast and made it known. There are so many Boxsters/Coxsters and Carreras out there sitting pampered in garages. Preferably one from Southern California. If you want the best deal on a Porsche you have to be willing to travel a little. But the last thing I would do would be to go for a car at a dealer with a built-in expense of the middle man's commission. If this were a car that's become hard to come by then the broker makes sense.

P.S.
I think you could get $10K for a base but only if you've done some of the big major maintenance jobs aside from the IMS. But then again, if your car looks purty there's always a first time Porsche buyer who will often overpay because he went nuts for the car after the test drive and doesn't want to bother with waiting or kicking tires. Water-cooled Porsches have very wide ranges on resale.