The ASX will make a good trailbike, but you are not going to win any uphill races on it. It will be really disapppointing on flatter XC trails as well. It is more suited for slow technical climbs and nasty descents. You will kick some tail when you point it down. Not sure on size.
Getting a dropper post is like getting a bidet. I didn't know I needed one until I get one and boy, does my ass thank me.
I tried one out at my LBS and to say its sluggish is an understatement. I bet if you pointed down hill it would be a sweet thang. For me I could not imagine using it as my everyday trail bike. Yeti really needs to put out something b/w 575 and the ASX
Should work out fine. Go for the bigger size since this will be a more xc oriented ride for you.
Another great option is a Santa Cruz Bullit. They are currently out of production, but will be back next year. They are also easy to find used. They are a bit lighter than the asx. If you do a search for "trail bullit" you will come up with a lot of ideas and suggestions from others. I spent all last season on a bullit as my all around bike. It worked great, climbed better than my heckler with the same Z1 fork. Only reason I sold it was to finance a new toy.