I think you're right about there being too many variables. First of all, do you have an idea of what size bike you need? You can drop 10k on a bike, but you're not going to ride it very much if you're not comfortable on it.

My suggestion is to visit a couple of lbs's and test ride bikes in different sizes and components. I say that because for 800, you should be in the market for their entry level bikes or maybe a holdover form 2010. You never know what you'll find. You'll also get a feel for their shop in case you need repairs or maintenance in the future even if you don't buy from them.

After you feel how much smoother 105 or ultegra shifts when compared to sora for instance, then you'll have to decide whether you'd rather have older higher end components or newer lower end components. Either way, it won't make you go any faster.IMO, a well maintained older higher end component is just as good a buy as a brand new lower end component, but that's a matter of personal preference.