You didn't specifically ask this but the single best accessory I bought for my vita was this;
Makes extended gameplay a joy. Love it.
On point two, there's one thing to keep in mind: Yes, you can re-download games you've purchased over and over, but that doesn't save your saved games. Think of the digital file like a memory card, with the saved game embedded into it. So, to keep your progress, you will need to use the Content Manager to copy the file over to your PS3 or PC, and copy it back when you want to continue your game on the Vita.
Also, Playstation Mobile games don't appear to be able to be re-downloaded, so those 99-cent mobile games in the Mobile section of the store can't be re-downloaded once purchased.