I've bought several of tops from them; one for the Boxster and and a couple for a prior car. They were high quality and workmanship. I don't imagine the glass window boxster room would be any more or less complicated but yes, it can be done by the average DIYer. They come with thorough directions and color photographs, at least mine did.
Your existing headliner should go back in place and does not effect the function of the roof. I still haven't gotten around to putting mine back though.
