Iguana's first encounter...?

Here in the near future I am getting a fairly young igunan from a pet store (Question #1) How can you tell how old they are if the pet store doesn't know, any estimates or something? (Question #2) I plan to get a about a 20 gl. tank for starters, is this adaquate? (Question #3) As far as handling it upon the first encounter should I wear a thick glove for the bite or a tail whip? (I ask this because my roomate owned a Bearded Dragon and i decided to try and pick it up, and it bit the hell out of me, and i still have a scar.) (Question #4) I know they are Herbivore, but whats there favorite for training purposes? And the last question would it be OK to throw a bunch of their favortie greens a blender and kick it on a couple times for dessert? Thanx a bunch...