She's started to show some interest in going from the cruiser to the mountain. I'm looking for suggestions on a solid, inexpensive (around $1k), used 5"x5" trail bike. She's about 5'4" and is looking at it more from an "increase her cardio" mindset. She has absolutely no interest in getting any air time. I was initially thinking along the lines of a SC Julianna or maybe a Heckler or something, but even used those are pretty pricey. Any ideas?