Hello Retroman,
from what i know about this transmissions your mechanic is absolutely right.
It's just a 6 hours ride to get a new "old" AT.
https://www.google.de/maps/dir/Bastrop,+Texas,+USA/Oklahoma+City,+Oklahoma,+USA/@32.7662726,-99.6620091,7z/data=!3m1!4b1!4m13!4m12!1m5!1m1!1s0x86449a5be8d1db 69:0x340d3c665badd960!2m2!1d-97.3152701!2d30.1104947!1m5!1m1!1s0x87ad8a547ef8d2 81:0x33a21274d14f3a9d!2m2!1d-97.5164276!2d35.4675602
But make shure to clean the valve body and check valves and valve filters. I've done this with MB 722.6 AT – which is quite the same than the Porsche AT. Not that complicated; especially if the transmission is out if the car. Also remove all old ATF oil in the transmission and torque converter - around 11-12 litres and put new ATF oil and a new ATF filter in.
The Mercedes 722.6 have sometimes problems with the electronic circuit board on top of the valve body. Don't know if there is a problem at the Porsche AT too. Picture shows MB AT 722.6
Regards, Markus