If the bike is stock and you want to go cheap, just buy yamalube brand 2R oil at your local Yamaha dealership. When people notice your question you will probably get different suggestions because everyone likes to use something different. Just search the forum, because the question about oil has been answered several times. My suggestion is just find a local bike shop, if not a yamaha dealer, that deals with atv's and see what brands they sell for banshee's. You can also order some online if you want, or if you have too, at sites like rockymountainatv.com or motosport.com. I personally am using Maxima Super M right now. Other good ones are Bel-Ray and the Klotz brand. I will also suggest that you get a measuring cup like the one on motosport.com called the Ratio Rite. I always mix my fuel like I'm in a lab because the morons who just guess and mix what they think obviously don't care and their bikes run shitty. If you mix your oil right, your bike will appreciate you for it and it will run great. Oh, and ALWAYS use premium.
As far as the aftermarket Cdi box is concerned, its purpose is to map your engine differently to take advantage of aftermarket upgrades. There really is no point in getting one if you have a stock exhaust system. Any shee fanatic on this site will tell you that a good exhaust system is the first thing you EVER do to a banshee if you want it to go faster, besides rolling it down a cliff. It is an unwritten rule. If it is not stock, there are still other things that are better that you can do before a aftermarket cdi box. You just need to snoop around in the mod section and learn.