Board size depends if you're looking for an all mountain board or a park board. For instance, I have a Capita all mountain board size 159, and I have a Forum Youngblood Doubledog 155wide park board.
Go by your weight, not your height. There's size charts anywhere, just search.
Definitely get your stuff in the off season. It'll save you hundreds and hundreds.
People start to age out of riding later on, it all depends, like me I'll be riding probably until i'm 60, and I'm 24 now.
For gear, you'll need: (i'll list a few prices, and how cheap you can get them)
-Board (around $300-600, off season probably around $150-500, depending)
-Bindings ($100-300, off season probably a little cheaper, not a whole ton)
-Boots ($150-400, off season like $80-200)
-A good jacket and pants (total for both around $200-500, off season about $150-300)
-Gloves ($30-70, off season a little bit cheaper)
-Goggles ($80-250, off season about $50-200)
-A good base layer
-A helmet if you'd like
Again, all those prices are just a guess, I've purchased a lot of stuff during the off season at a ridiculously cheap price. And there are sales on a lot of sites during the season (from last seasons gear, and just sales).
Check out some of the sites
My favorite is the-house.com
Best of luck to you!