Hey Joe, welcome to WF! If you are wanting to go with 32s, you can likely get by without much lift at all, maybe 3/4" and wheel spacers depending on the wheels. If you do want 2-3", I would suggest a 2.5" BB (budget boost). Daystar and Teraflex are a couple names that come to mind. With a 2.5" BB, you can stuff 35s under the JK.
Rubicons come stock with 32s and you can usually pick some of those up pretty cheap. There are not many 15" wheels for the JK. Without the proper backspacing, the wheel may hit the calipers.
As for MPG, I have no idea and I have 32s, but you could probably look up a window sticker for a Rubicon to find out.