And next year put the battery on a maintainer over the hibernation period.
A list of batteries and their sources/prices is at
http://mike.focke.googlepages.com/optimabatterymountingkit.
I listened to a gal get hustled at another make's dealers for $168 installed, I needed one too and mine for the same car cost $60 with a better CCA and a better warranty.
You need not go with a Porsche branded battery, in fact you are throwing lots of $ away if you do.