Upload & Sell: On
I would focus on the software. Make sure the app you get lets you the things you want. It is pretty amazing that some apps do not do basic things like let you specify the order in which images are displayed. Instead they display on last date updated, file name, or some other crazy setting. Also make sure you can do albums and if possible share images across albums.
As far as hardware goes, the new iPad has a great screen. Your storage requirement depend on what you are displaying and how much. I ensure my slide shows are not too long since most people do not want to see more than a dozen or two images in an album. Because of this I find that storage is not too big of an issue for still images. If you plan on dropping in video clips then storage is more of a concern, but most people only want to see short segments of video. I have a 32 GB new iPad and have tons of spare space.