Thanks for all the info guys. All this is helpful. I emailed another local bike shop and they have some Bianchi bikes with the Shimano 105 parts. I believe it's an older Bianchi but maybe I can try and get a deal off of them.
Wow, cool to see a bike community on slap. I started out cross country mtbing with my dad from about age 6 to 12, when I started skating and competitive road cycling for years. Im now back mtbing again and its probably my favorite thing aside from skating. Given their parallels, such as having the ability to choose your own lines, pumping, getting air and doing drops and gaps. I would wholly recommend you consider getting a mtb rather than a road bike if their are trails in your area?
I worked in a bike shop for 3 years as a mechanic/sales person and what the others have said about bikes holds true in my experience. Newer Tiagra is actually almost identical to the 105 5800, which I had on a TT bike 5 years ago, it never skipped a beat but new 105 is still going to be better. Sora is still going to be worse, esp. Front derailleur.Tyres are also the first thing you should consider upgrading, followed by wheels, bars/stem then saddle. Continental gp4000s2 were always my favorite. Gatrorskins are better if you ride in a city. A good pair of bike shorts is also essential, dont be skint with what you buy, your balls and sciatica will thank you for it.
Buying a new bike from a bike shop is good for two reasons, you know the bike has not been mistreated and you can form a relationship with your mechanic so that your bike gets the best possible care. Aside from that, if you can be bothered taking the time to learn how to maintain and repair a bike, you are going to be much much better off. Not only is it easy to learn, it is quite therapeutic and will save you a LOT of money.
However, if you want a wayyyy better bike for the same price as your $850 bianchi, (bianchi has had a long run making bikes and has sponsored many of the greats btw, good choice) you should consider secondhand. The two downsides to that are that if you take the bike to a shop for repair they wont be as enthusiastic to see you, (like getting your local to grip a deck you got online) and you have no idea what the owner's maintenance regime, riding style and mechanical sympathy were like. If you like you can post pics/listings on here and I will do my best to see if the bike looks in good shape, there are a few tell-tale signs if someone doesnt give a fuck about their bike and doesnt clean it. That's not going to be a substitute for looking at the bike fr though.
Finally, do try and get a deal off them if you buy new, negotiating a price for a bike was honestly the best part of working at a bike shop for me. And without going into specifics, the margins on bike stuff are huge so there is almost always room to get a better deal.