In the LR3, I went through a full tank of gas, but did not need any more than the one tank. I did bring a Jerry can though, and so should you. It's about 140 miles of slow dirt driving, much of it in deep loose sand, where throttles are happiest buried into the floor matts
Fuel up at the Avi casino and then again in Yermo near Peggy Sue's. There is NOTHING in between unless you pull off near Soda Lake and visit the Mad Greek.
Temps range wildly. It can be HOT, it can be COLD. Mid Hills campground is 5200 feet, so it's not uncommon to get below freezing in spring and fall. I was there in January and it snowed about 4 inches on us. You need to bring a wide assortment of clothing for any desert adventure.
The water crossing at Afton Canyon is no problem in the LR3. It's a right fine little submersible. HOWEVER, you do not plow into it at speed! The proper technique to go slow enough to keep a bow wave in front of the grille, and NOT over the hood. A steady speed is far more important than total speed. This keeps the water below your intake on the side of the vehicle and generally out of the engine bay (air is trapped under the hood creating a nice little pocket of air to keep the dainty bits dry).
Make sure you have this map AND book (get both, do not skimp):
http://www.desertusa.com/web_cart/db/pages/5094.html
If you post up on ExpeditionPortal.com that you are going (in the CA/NV subforum), you will have plenty of folks interested in joining you...or you can contact LRCSD.org or SCLR too. Folks are always wanting to take on the Mojave Trail. It's an easy road, but lots of fun.