creating a multisite from existing websites

I manage websites for two small non-profits. They were stand alone sites made with Rapidweaver. We have decided to move to the Wordpress platform so that members of those non-profits can take care of their own CMS with folks who use PCs as well as Mac users.
I have set up a Multisite with three sites at present - both of the non-profits plus a site I can use for test purposes since I am still learning Wordpress myself.
Now that I am a premium member here, I hope someone can help me choose the theme(s) that would be best for these sites. We need to have static front pages, the blog pages would be for news and notifications to their memberships and would have guest hosts (bloggers) occasionally.
They need to be able to set up to receive donations through Paypal. One of these non-profits has two books they want to sell through the web-site, too.
The one with the books to sell is also going to be very content heavy with slideshows, galleries and audio content as well as text.
The other will be more general with less content, but will want some bells and whistles too.
Is it best to have one theme for all the sites in a multisite setup or can you have specific ones for different sites?
Are there different themes that would be best for the needs of the three sites?
Are there tutorials that would be good for me to watch that would address some of these issues?