I agree with huckones, the hunting shows give a false impression that every time you toot on a call a monster buck will come running. It isn't like that at all and actually pretty far from it.
If you buy anything buy a grunt tube, it's probably the most used go to call. A snort wheeze can be made without a call and you can make your own rattling bag if you don't want to spend the money. Cut 6 or 7 pieces of a pine 2x4. An old 2x4 works best as it will be dried out more than a new one. Make each piece about the size of a hot dog, they don't have to be round but just about that size. Put them longways in a sock and there you go....a rattling bag. It actually sounds pretty good. Or you could always just find some real antlers and use those.
Don't call too often and only use the right calls for the right time of year or else you'll just scare the deer and educate them at the same time. This article has some good tips.
http://www.trophyhuntingobsession.com/8 ... ling-tips/ I'd buy the grunt tube and possibly all 3 of the calls mentioned in that article just to get started. Good luck.