Congrats on the rifle and at a fair price too. I'll attempt to answer your questions. It would seem the rifle is the cheap part compaired to what a guy can spend on gear. I would check for used items every place I could think of. If you know how to get in touch with other shooters, don't be afraid to ask if they know any one who has used gear.
I understand Black Hills makes match ammo from 52 to 80 gr. My AR's have seen very little if any store bought ammo.
The scope should be no more than 30 power. I know there are varible scopes out there but 30 is the most you will need. Any thing much higher isn't going to help yyou see unless there is proper lighting which seems rare. My Kowa has a 27 power eye piece. You do what as wide as front lens as you can get. 60mm should be the smallest for the front lens. Champions Choice has a good selction of cheaper scopes. Also you will want a 45 degree eye piece.
Scope stands have gotten out of hand. No body builds a good reasonably priced scope stand IMHO any more. I would get a 3 legged stand with a 1 inch rod as a first choice. They have to be tall enough to get the scope high enough to look through the scope while standing to shoot off hand.
Shooters stool. You don't need a war wagon with wheels. A good ole' stool will more than work. I like the big stool from Creedmore.
Mat. Make sure it's got rubber on the shooters end.
Coat. Creedmoore. They will build you a custom coat. I like the pads, you don't really need the fancy zippers on the back so you can sit. I know some who have these, mine dosen't. Mine works just as well as my friends.
Glove. Well the $18.00 one works as good as the $38.00 one.
www.creedmoorsports.com
www.champchoice.com
http://www.ray-vin.com
www.okweber.com
I'm sure there are more supply people out there, but these are the one I use the most.
Positions: Check
http://www.highpowerforum.com/ They have a good thread on proper positions with pictures.