Thanks for answering the questions and welcome to DRN.
Okay so your size and where you will ride (desert) calls for a 450. I'd suggest looking at something along the lines of a Honda 450X, or anything else made more for the riding you are interested in. Most of the major brands make an off-road version of their MX bikes.
These bikes will typically come from the factory with wide ratio transmissions, larger fuel tanks, suspension tuned (usually softer) more towards trails, headlight, etc.
As far as the power of a 450 goes and a new rider. Everyone will have their opinion on this but at the end of the day and what type of riding you are interested in, you'll just need to take it easy until you get some time on the bike and get comfortable. 4-strokes are very controllable and have linear power delivery, so taking care with the right wrist will keep you safe.
You can also look at what Kawasaki calls "Recreational Trail Bikes" . These are far less capable in general, typically heavier and much less powerful. Examples are Kawasaki KLX 300, Honda CRF250F, Yamaha TT-R230 and others. These things are under-sprung to begin with and typically have a lower seat height, something not well suited to your size.