I just finished upgrading my craft from Vision to Vision Plus, and even though it took about $450, it was fun, educational, and cheaper than buying another craft completely.
If you want to go all the way to the Plus model, you'll need the 3-axis camera, wifi module, respective wire pack (part 8), and Vision Plus NAZA control module.
It's a blast when you get all that hooked up and working by yourself, trust me.