Don't jump start. Open the rear hatch, disconnect the ground strap where it attaches to the frame with a wingnut behind the toolset, hook the negative side of battery charger to the disconnected ground strap, positive to the jump post, charge the battery. Remove the charger, start car. I use a 20 amp charger and it takes about 20 minutes to put enough of a charge on the battery to start the car.
Not much spread in cost between cheapest and best LH rebuilder, I would use the best,
Porsche Electronics Specialist - JDSPorsche or Rich Andrade (don't know his details). Roger at
Home - 928's R Us Online Parts Catalog can do an exchange for the jds units.