a) I would go for download every time as streaming can be a pain even with broadband, especially if the site is busy.
b) Yep go for both the more choices the better.
c) One again I would go broadband especially if you are talking about very large files.
d) How to choose which to go on first, oh boy what a choice you sure we can't have them all
A good preview section can make a lot of difference between joining or not so better the previews the better chance of people joining.
A one day sample membership is also good as it give you a chance to see that the content is as good as the preview, which is not always the case.
Choose a video format that will play in mediaplayer and not the dreaded Real player.
Good luck with the new site I will have to check it out when it is up and running.