Meg, good luck with the dentist! Hope you succeed in being able to sell some of your cards to offset the costs. One hint -- send cards to your friends or co-workers (if you have co-workers). I don't usually sell my cards, but I have found a lot of people asking me if they could purchase my cards, so it is sort of a cheap way to advertise your talents. Another idea is to offer card sets to local charities that might have silent auctions. It goes with your idea of supporting the charities, but if you have your contact info in with the packaging, you might get a call for a pay-for job.
The SCS forums are great places to find out which equipment are gotta haves and which are disappointments. Since everybody is different, I would suggest finding out WHY people love the products they do and checking out their gallery to see if they seem to have similar style to your own. This will help you know which suggestions are likely to be the most helpful for you.
Participating in the challenges is definitely a great idea.