I've had my Appy for about 9 years now and he's probably the hardiest horse I've had. He's never had problems, except for the occasional scratch and what not. Their usually known to be pretty hardy...like I said he's almost 16 years old and has been showing constantly for 8 years. But the appy's are known for Moon Blindness of which is not life threatening but still something to watch out for.
Now my appy does have a thin mane and tail, but I try to let it grow a little more then usual and then pull it, and for the most part it's stayed fuller. But his tail is a different story. It's always been on the "ratty" side...almost reminds me of an elephant tail in the summer! LoL...anyways as far as gait, it seems to vary. My appy is actually really smooth in the trot and canter suprisingly, and has a longer stride then most do...but their all different.
Congrats on your purchase though...hope everything goes well for you two!!