I do not recommend trying to rig an 18" wheel. You will have to also rig a way to install a brake, and the rear tire is not really that much wider than the front anyway. If you want to build a supermoto (definitely not recommended for sand) you can get 17" wheels for the XT, and run street tires on them. I have gotten my XT stuck in sand in AZ, but always managed to get out. Now I stay away from sand. I have been tempted by the TW200, but there is no way I could fix a flat on one of those huge tires by myself out in the desert. Also, my XT has an aftermarket centerstand (no longer available) they have never been available for the TW200. I watched a long time TW200 rider changing a rear tire at home in his garage, with a bike lift, and he really struggled with it. I have already had one flat rear tire on my XT late last year. Took me about 3 hours to fix it. Would have been impossible without the centerstand. As far as the jets and ethanol gas goes, yes that is an issue. Every time you finish riding, shut the petcock off and run the float bowl dry. That will keep the ethanol away from the jets. If you are going to let the bike sit for awhile, I drain the float bowl, remove the fuel line from the petcock, and fill the float bowl with Seafoam or Chevron Techron to clean the jets.

The above is my opinion. Your mileage may vary.
1994/2001 custom built XT225 with a ton of aftermarket parts.