Lots of questions going on here, but here's my $.02
1) Yes, the 986 and 987 have an integrated "roll bar". It's what makes the Boxster a very safe and stiff "open" car.
2) Track driving DOES cost money ( both car prep, driver prep, hotels, gas, lodging.. tires, brake pads/fluid etc) - It's not unrealistic to budget $500-?? for a "weekend" at the track)
3) Got a $10 head, get a $10 helment. Good helmets can be $300-$1000. Try one on. Try another. Make sure it fits snugly and with comfort. Too loose = no good. Too snug = headaches. Also make sure you can fit your glasses on (if you wear them) and maybe have room for an earpiece, as most instrutors use helmet to helmet communicators rather than screaming at you all day.
4a) As of yet, Brey Krause does NOT make an extension for the 987, only the 986 cars.
4b) Some tracks, like VIR, REQUIRE a Boxster to have a BK bar, even if you are well under the "broomstick" level.
4c) Some clubs/tracks waive the broomstick requirement IF you run with the optional hardtop in place
4d) Some don't even know what a broomstick test is
5) Insurance... Case by case. Get a copy of your policies EXCLUSIONS first. It may already be spelled out. Yes DE is not racing, but with DE's popularity, it has come under fire from Ins. Co.s Oh, and even if the cover it the first time, it won't mean they won't drop you come renewal time
6) harnesses - when used in PCA events, will require a seat MADE for harnesses. Your stock seat will not pass this test. So, add special seats if you want to run harnesses.
7) If you car is street registered, you will need to maintain all factory equipment to pass your state's inspection programs. Don't bother removing them.
Hope this helps!