If you were to go with a 20 inch wheel, the offsets would be the same for each width. Most of the tire widths would still be applicable, but you would need to check with an expert first. I assume the less the sidewall, the less room you have to vary tire widths and still seat the tire properly.
As far as tire sizes, 245/30R20s are about 25.8 inches tall. For the rear, 275/30R20s are about 26.5 inches tall, while 275/25R20s are about 25.4 inches. I would opt for the latter personally, as I would prefer a slightly shorter rolling radius vs a slightly longer rolling radius. The stock tires are right around 26 inches tall.
Realize if you go with a 20 inch wheel, you will be increasing unsprung weight/lowering performance...at the expense of a little bling bling. Many prefer this as it does look nice, but personally, I am considering going with an aftermarket set of 18s for weight saving purposes.