I picked up an absolutely beautiful like-new Trek Madone (carbon fiber without a single scratch) some years ago (for $800) and have put upwards of 5,000 miles on it without a single problem. In fact, I'm going for a 20 mile ride on it this morning.
Side note: Many young couples buy beautiful his & her bikes, do a little riding, and then the babies start to come. Bikes gather dust and young family needs some extra cash for all those baby things.
Bottom line: There are a ton of beautiful barely used bikes out there.
As to Shimano 105, it is the workhorse of bike components and over the years has benefitted from "trickle-down" technology (where the tech of really high-end bikes becomes incorporated in medium price components over time).
I recommend against spending big bucks on a first bike because you really don't know your style of riding and what your preferences will be. On top of that, many of your preferences can be met with after-market equipment.
Side note: I have a good friend who bought a new bike couple of years ago (hadn't ridden since childhood). He studied every angle and shopped and shopped. He decided to buy a new bike for around $3,000. It had every bell and whistle that his "analyses" told him he would need. I offered several of my old bikes to him so that he might ride a bit and get the feel of the road. But no...he said he would just wait for his new bike.
Well, he has put perhaps 50 miles on it. He found out that biking is painful. The seat hurts his butt (that usually goes away) and, he thinks, his bike is too long for his frame.
Point being, there is no--I repeat, no--substitute for miles in the saddle in determining if a bike is right for you. Ergo, buy a nice used bike for very little money and learn the sport.