I never service any of my cars outside the dealership, mainly because I've spent over $100K on each of my cars, and where I live I don't trust anyone but the dealer to maintain them.
But if you really want to work on a budget and you're sure that you can find a mechanic that's competent enough to maintain your car for you why not?
My Cayenne is 6 years old now and has never been serviced outside of Porsche and it still runs like that day I got it. No major issues yet and I couldn't be happier. I bet I could keep this car for another 6 years easily if serviced right. That being said It's only hit 89,000km (55,302 miles) yesterday..so I don't really drive it that much.
That being said though I got a package with my cars when I bought them here, for both cars I was able to get 5 year free service (not including parts)... I know it may not seem like much..but it's awesome and I was able to renew it on my Cayenne by extending the package for another 5 years by paying a small fee.. so maybe look into that and see if it is offered in your country.
Other than that man, the car will let you know when it's ready to be serviced. From what I remember I think it's every 15,000km (9,320 miles)? Basically once a year for me at-least, then again I don't drive too far. I remember Porsche telling me that the service due can fluctuate depending on how the car is being driven, so if you're driving a bit harder than most people you can expect to be going to service more often than most people... I'am not sure how accurate that is, but I was told this since I was surprised I had to service my car twice one year. I believed them though since that year I was driving a bit harsher then most drivers.
So secret? Maintain the car well with either porsche or someone competent and knows what their doing. Check with your local dealership if they have the 5 year free service package, do that for some time and once 5 years pass find someone else to maintain the car or extend it.
Good luck, a 3.5 is not difficult at all to achieve mate, just put in the time and it'll be worth it.