Alright, so, bone and muscle strength are obviously a must. Though if you have inherently brittle bones, I wouldn't recommend tricking to begin with. Muscle strength is handy, but you'd want to focus on core strength to get most rotation-centric tricks pat down. Leg muscles too perhaps, if you're a little thick on the side and still want to get in on that tricking action.

Depends on what you consider enough. If say, you want to do aerials and not fuck up most of the time, you'd want to be able to do a rather wide split, etc etc.

mmmmm, i'd say yes based on experience, but you'd want to do your own research on that

I really, really do not consider myself a flexible or very strong person. I can't even touch my toes, I reach a little over halfway down my shins. I bench, at most, 90 pounds. Yet, I'm able to do quite a few lower-level tricks, such as backflips, frontflips, websters, and bkicks. And a few more on trampolines.

The point is, you don't necessarily need to be super toned or flexible to start tricking. Hell, the more you practice it, the more you'll get in better shape anyway. Just stick to trying not-so-insane tricks, get over the initial fear, and focus on technique. It's fun, and overall worth it.