Hi - just over a year ago, I sold my 96 Miata that had only 40k miles and bought my current 2002 2.7L Boxster, also with only 40k miles. I think I have a decent perspective on this. There is just no question about the reliability of the Miata and its low costs. Some say there is a "smile for every mile" that you drive a Miata - and I cannot deny that. But I think this question is comparing apples to oranges. The Boxster is a Porsche, period. The Boxster is as solid as you can get, plenty of power, tremendous handling, and its a Porsche. The Miata is a bit like a toy, the Boxster is a "mans car" which may be an inappropriate analogy these days, but it's the truth. And that's the difference I feel when I drive and people stare at me. I also believe that there are many DIY possibilities on the Boxster, which will lessen the annual costs and not be terribly burdensome either, especially with the mileage that you plan to put on it. You also seem to be looking for the right things to be done on the car, IMS, RMS, etc. Very smart. So, I say go big or go home! C'mon guys, just having some fun!!
Good luck.