So you don't really know about the platform, have never really assembled a small platform rifle (you stated package deal, I'm assuming some sort of kit), and you want to do this on the absolute minimum budget possible.
What could go wrong?
Are you mechanically inclined? You have an 80% lower, did it come with a jig? Do you have the needed tools to assemble a rifle?
Your post has me thinking you are a novice. This in itself isn't a bad thing, but I can tell you that you should have read more and spoke less already. There is plenty in the forum to answer 95% of all your questions. Start there.