As it's not warrantied, then see what a decent Porka specialist can do the service for. It's not like an OPC service history adds anything other than black inked stamp mark in the book, which certainly won't really be worth the additional cost in your case. Unless of course the indy price is close, then go for OPC.
If I was in the market for a GTS type car then a decent indy service would probably be more impressive in my eyes, rather than just an OPC stamp and invoice of £lolz£.
A Macan is nice, but they really arn't that big. Why not just drive and enjoy the GTS for years to come and get a cheap SUV load lugger?
An old 955 Cayenne Turbo would be a very sensible choice