I have a 2005 and haven't had any issues, and there aren't any big known issues. Some minor updates here and there.
I think the Boxster S makes a reasonable good track car. Like any other car there are a number of upgrades that can be made to improve performance but it's a solid platform with great balance.
The maintenance intervals are 20K but if you track the car you would obviously need to maintenance the car more. The oil changes, brake pad changes etc.. are very easy to do so you could consider doing some maintenance yourself.
I have not heard that tracking the car voids the warranty. My dealership preps cars for track days etc.. and have never questioned the warranty. Porsche makes these cars for the track. Now if you go into the red frequently, that's a different story.
Just a thought, but have you considered the Cayman S. That would get you up to 295 HP with a better structure (solid roof). There also seems to be pretty good deals on Caymans lately. The Cayman owners seem to be more serious with track use than Boxsters (in general). There is a good site if you want more information
www.caymanclub.net
Regarding options, that can be personal preference but the general consensus seems to be windstop, Bose upgrade, heated seats, sport steering wheel, and xenon headlights. I'm not convinced the PSM or Chrono are worth the investment, but thats probably a whole different topic.