Depends on the type of hunting you will be doing. Are you deer hunting from a stand?
If so, being comfortable will allow you to sit longer, which will increase your chances of success. I agree with surplus BDU clothes. I have a closet full of Mossy Oak camo, but I still sometimes wear surplus pants. If you splurge on anything it should be a good warm pair of boots and good socks.
Rocky and Lacrosse make good boots at affordable prices.
Wal Mart is a good source. You should also order the Cabela catalog, you can find about anything you want in there.