Bruce: This is definitely an easy DIY, and a completely different procedure from that of most other cars, which require getting under the dash and in front of the console. Not so for the Boxster, as
everything you need to get to can be accessed from outside the car, below the bottom of the windshield.
I did this a few year ago but did not do a write-up because it was so simple.
IIRC, you have to remove the wipers, the black vinyl apron at the base of the windshield (you'll have to pop off the small circular plastic plugs below the left and right corners of the base of the windshield), the wiper mechanism, the two cross braces that run to the strut towers, the three cosmetic covers over the battery area, and the battery. After that, you will carefully peel off the foam cover that seals the heater core compartment, pull two clips outboard and remove two screws, along with disconnecting the "in" and the "out" small diameter heater core hoses from the two plastic nipples on the heater core. That's about it from memory.
The heater core is only for heating and only has coolant in it. You will have to bleed the coolant system when you are all done.
DO NOT GIVE THAT MECHANIC $1000! You will kick yourself after you see how straightforward this job is, and not nearly as bad on your back as in other cars.
Here are a few photos that might help you. I do have a bunch more if you have a specific area you are not sure about.
Once you remove the black vinyl apron...the two hose pliers are there to minimize the need to bleed the system.
Remove this cross-member and the one on the opposite side.
Part of the vinyl cover (peel off carefully), other end of one cross-member and part of the windshield wiper mechanism.
Pull outboard on the two clips where the yellow arrows are pointing after you have removed the two screws at the red arrows. Then you can just pull up the radiator core to remove it.
Once you remove the heater core, you will probably see coolant at the bottom. Clean it out thoroughly to get rid of the aroma in the cabin.
Regards, Maurice.