100 MB for downloads is plenty I think, even most hi rez stock photography doesn't exceed that size.
Are you able to test the purchasing transaction portion using fake products and with the store in test mode? That would be the only other thing remaining to fully test everything. They way I've seen it done before is just set the price of some of the products at a penny and run an actual purchase.
Yes 100MB would be plenty for stock photography.
I have tested the purchase transaction in beta and it works perfectly. You will receive an email telling you that there was a purchase on the site and your customer will receive a receipt just like all other e-commerce sites like Wal-mart or Amazon. Right now since I am running in beta it allows the transaction to take place without actually making a true transaction using a payment gateway. The other nice thing about My Cafe Commerce also is that it allows for inventory control which is something most e-commerce sites need.
Hope that helps and if you have any other questions just ask and hopefully I can help.