To fit 31" tyres you will have to do a lot of trimming, and raise the bump stops on the front axels. You will also have to flatten the seam on the main body behind the front tyres and possibly flatten this area back a bit as well. As for 15 or 16 inch rims - swings n rounda bouts. I've found most tyres I've fancied are available in both sizes. If you go 16" then Vitara steel rims can be picked up cheap enough. I've also heard that Diatsu Four track 15" rims give better off set. I've got 15" 8 spokes on mine. The biggest issue you will have with 31 " tyres is gearing. %th will become redundant and even 4th hard work on motorways etc unless you fit either a rock lobster SJ conversion or a Trial Jimny gear set.