Back in 1988, I bought a new Trek 830 MTB. It had the U-brake and was a very solid bike that lasted until just last year, when I finally broke a drop-out. I strongly suspect it was the result of clamping the bike into a trainer the previous winter, and not the result of riding. I bough it for road use and so it was very tall, with the horizontal top tube.
Anyway, I had several types of fenders on it over the years and had no problems. My tire sizes ranged from 2" down to my favorite 1-1/4" slicks. I miss that bike. It had all the attachment points of a true touring machine and was extremely comfortable for long road trips when fitted with different bars and saddle.
I recommend finding an MTB that has the Shimano BioPace or Sakae OvalTech chainrings. Sheldon Brown has a good write-up on this on his site.