Congrats on the new car. I've never driven a convertable so I can't really comment on the blind spot, but with the size of the 3 series mirrors how can it be a problem.
I haven't actually heard of anyone installing anything other than the OEM alarm. (although I am sure ppl do) The car is prewired for it so its an easy install. You can do it yourself. You will need to have the dealer activate it for you, though. So if you DIY then make sure you are friendly with the dealer.
