Bolt pattern is 5x114.3
18's will have a smoother ride than 20's because you will have a taller side wall on an 18, which will absorb more imperfections in the road. I would go with an 8 or 9" wide wheel in the front, no wider than 9". For the rear you can go from 8-10". 9" front and 10" rear is a good staggered size. Not sure what the max offset you can run. Here is some information on wheel sizing Wheel sizing - Wikipedia, the free encyclopedia
18's will have a smoother ride than 20's because you will have a taller side wall on an 18, which will absorb more imperfections in the road. I would go with an 8 or 9" wide wheel in the front, no wider than 9". For the rear you can go from 8-10". 9" front and 10" rear is a good staggered size. Not sure what the max offset you can run. Here is some information on wheel sizing Wheel sizing - Wikipedia, the free encyclopedia