How can I research whether or not an online community is willing to spend money on products that are within their area of interest? Is there a better way to determine that they will spend (or not) other then to create an MVP?
Not everyone who signs up will buy, but you can be sure that if you only get a few signups there isn't a market and if you get hundreds of signups your are on to something good.
I recommend hooking this up to someone like MailChimp (or one of the other e-mail marketing companies), this will both make it easier to send announcements later and let them filter out signups by spambots and other garbage.