I'll turnabout a post to yours since you graced my sore knees question with an answer. I purchased a United 24" with a Kovachi wheel (Suzue hub, 13 ga. spokes, Sun rim) and a 24x2.125 tire with a max psi rating of 40. As I mentioned, I am about 285 lbs. and have been riding for a couple months. According to John at Unicycle.com, a Kovachi wheel is one of the best around. I don't do any major drops but have done some hopping and riding off curbs and haven't had a problem. The other secret I've learned is that most unicyclists usually run with more pressure in the tires, up to 100% at times. Because of my weight, I pressurize my tire to an additional 50% or 60 p.s.i.
Hope the info helps.