First and foremost, congrat's on the little one. Health, prosperity and joy to you and your family. Children are the truest blessing. I will share with you my sort of similar story.
I sold my custom ordered Carrera S when we were blessed with a baby boy. As much as I loved the car, the reality was that I was just never going to drive it because rear facing car seats just don't fit in the back of the P car and the times when it would be just me and my wife were/are few and far between. So I sold it, pre-funded my kid's college savings and haven't regretted it for a second. All my family and friends who know what a hard core sports car guy I am thought I'd lost my mind but I can honestly tell you that next to marrying my wife and having my son, one of the best decisions I've ever made.
My son is nearly 2 and a half and in a proper, front facing car seat which opens up a lot of options. So a few months back I started thinking seriously about buying another "fun" car for me. It would be my third car (we drive a 2010 Audi A6 wagon as our daily driver and we have a Honda van that my mom (our child's caretaker) drives) but I wanted to be able to take my son with me.
I remain an active PCA member and I've always loved the way the Cayman drove, just not the way it looked. I was invited to the launch of the new 981 Cayman by my dealer and took it for an extended test drive. I was absolutely blown away. It's honestly all the power you'd ever need on the street with absolutely sublime handling. And although I'm a hard core, borderline militant, three pedal manual transmission guy... I have to admit that the newest generation of PDK is really, really damned good. My dealer offered me a fantastic price on special ordering a new one but I passed.
Instead, I bought a 2008 M3 sedan. Frankly, it doesn't handle anywhere near as well as the Cayman and it's a 5-year old car with 42,000 miles so it's not perfect. That was a hard pill for an anal guy like me to swallow but my reality is that I paid less than half of what I would have paid for a new M3 or Cayman S and I now have a car that I am already sharing with my son. His car seat, stroller and his gear all easily fit. He loves riding in "daddy's new double oooh" and for me, that's worth it's weight in gold.
We each have to do what's right for us and to each his own but you're a new father. I don't know any parents who don't find that children change them and at least in my case, I can tell you with an absolute straight face that one of my great joys is sharing my passions with my son. Whether it's baseball, cars, or the ocean... nothing compares to the joy I get when I'm sharing those activities with my kid. I have to believe you'll find the same thing to be true.
The Cayman S is a fantastic car. Anyone who owns one is driving one of the great cars ever. But the M3 is a damned fine car too. Arguably the best small sports sedan ever. And you can share that vehicle with your child. You can't do that in the P car.
Best of luck in whatever you choose.
And again, congrat's on the newborn. Enjoy each stage of his/her life because you'll be shocked how fast they grow. Especially when you're sleep deprived
