If it is going to be a ground stand, go with treated around the base (use some cinder blocks to lift it up a little bit), but if it's going to be raised, I would go with regular 2x6's on the base and 2x2's to frame it out. Do you have blueprints already?
Everyone on here probably gets sick of me re-posting this, but I enjoy building a good deer stand. In my opinion, if your hunting experience in your deer stand is enjoyable, you're more likely going to want to sit in it more and for longer periods on time!
Here is a link from my build awhile back--
http://texags.com/main/forum.reply.asp?forum_id=34&topic_id=1694834&page=1&nomobile=1 That siding mentioned above is some good stuff! I'll probably make my next one with it. Just make sure you use primer AND paint (inside and out. I did 2 coats of each)so you don't have any wood rotting. Also, the more wood glue you use, the less "squeaking" noise you will hear on windy days in November!
Other helpful tips:
-go to Home Depot or Lowes and find their "Opp's paint (stuff returned) and get your outdoor primer there.
-use galvanized screw AND wood glue
-check into renting a airless painter. For a guy that hates painting, it might be worth the $40-60 for a days use!
-slope your roof so water never sits on it. And slope it away from your entry door. (a buddy of mine slopped his roof on his stand, but it was slopping towards the ladder he used to get in it...so if it was raining pretty good, water was coming down right on him! haha)
-use the metal/tin material on the roof (use the special tin screw and screw it to the 2x2 that is cut out like a "wave"
-find a local carpet store and go over there one evening and find some old carpet in the dumpster. Make sure it's not stuff removed from a murder scene, though!
-look on craigslist for used reclining office chairs.