Welcome. As a dyed-in-the-wool BMW guy myself (3 M3s, a 330 ZHP, a 335, a 535xiT 6M, etc), I'll just say that you will not regret buying a Porsche. Actually, you may regret the time you wasted before buying a Porsche, but you won't be disappointed.
A couple of thoughts on your post:
-If you are looking at '05s, you are actually looking at 987s, not 986s. The first year of the 987 was 2005, bringing with the 987S a slight bump in power over the 986S (to 280). The engine remained a 3.2L, and would stay a 3.2L in 2006; in 2007, the 3.4L from the Cayman S was fitted to the Boxster S, giving 295 hp (IIRC).
-One issue you'll want to think about is the possibility of intermediate shaft (IMS) failure. This is a valvetrain component that runs underneath the engine; failure = new motor = not cheap. If you search for this, you will find A LOT written about it, but the bottom line is that some unknown percentage of these engines experience a catastrophic and expensive failure, and there is no guaranteed maintenance procedure to prevent it or inspection that will pick up an impending failure.
-I won't try and summarize the various theories and remediation attempts here. Suffice to say that in my reading, as best as I could tell, the 986s were all vulnerable to this (though, interestingly, apparently the earlier 986s were less susceptible). The Boxster Ss after 2007 and all Cayman Ss (i.e., cars with the 3.4L engine) do not seem to be, or at least do not seem to be anywhere near as vulnerable. There is some suggestion that a running change was made to the 3.2L engines in the middle of model year 2006, such that the later 2006s (with the 3.2) are also not as vulnerable. No one has been able to absolutely confirm this,
AFAIK. I ended up buying a late-build '06 because I found the right car and I persuaded myself that it was better than an '05, but I consciously avoided '05s for the reason above. Am I fooling myself? Maybe.
-The 987 Boxster can go onto the track at PCA DEs without extra roll protection, so long as the driver's head is under a line drawn from the roll bar to the windshield header. I've tracked my car a number of times without a problem.
-Even the 3.2L S is enormously capable on the track. It's really quite a fast car; you can be lulled into thinking that the older, smaller engined, entry level Porsche S isn't as capable as some of the newer cars, and that's true to an extent, but the differences are a lot smaller than you might think.
-One thing that will probably astonish you coming from the BMW is just how good the brakes are. I was in a run group with a guy in a turbo Z4, and he was pretty quick...but he had to pit every so often to let his brakes cool. Even before I switched to Pagids for track duty, my car had no such issues.
-The Tiptronic is garbage (IMHO). Well, that's a bit harsh, maybe, but in the cars you're looking at, it's just a torque converter automatic, not a DSG; that came later. If it is at all possible for you, the stick is the much, much, much better choice. One of the most brilliant things about my car is the clutch take-up and throttle feel. It's so much better than even the best BMWs that I've driven that it's not even worth making the comparison. You would hate to miss that.
-There is essentially no difference in maintenance costs between the S and the non-S. The brake pads/rotors are a bit bigger (and thus more expensive) in the S, but that's negligible.
EDIT: When you get a PPI on a car that you are looking at, make sure to have the DME dumped and check for over-revs.
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