20's wont fit without rubbing, and the hastle involved is pretty sizeable. Spacers would pretty much be out since I doubt you;d find a 19 inch wheel with an offset high enough to prevent rubbing.

As far as replacing the axle goes, you are talking serious money, and I think it would be way cheaper and easier to get a 4 lug 19 inch rim in the first place.

If you really want to do the swap, look in a junk yard for a 6th gen V-6 as a parts car, and be prepared for alot of work.

AcsRcool is dead on about braking, but another thing to factor in is the better stopping you get from the usual high performance rubber mounted on plus size rims.