I downsized from the stock 18s to a set of black steel 16s. Yokohama Geolander 265/70/16. Overall 0.5" taller than stock to keep the gearing relatively close to stock since there are no direct replacement aftermarket gears... I wouldn't go to big on tires.

As for offset... do a search on wheel offset and you're bound to turn up lots of good postings from the past.