Well, Shimano (the well known fishing tackle brand) produce groupsets at different levels of cost, cheapest to most expensive:
Claris
Sora
Tiagra
105
Ultegra
Dura-ace
There are sub levels within some of the sets, but that is the breakdown. A 105 build is therefore considered a good quality...
I've bought a couple of bikes second hand and not had an issue. One from a friend and one from a Facebook ad. G-Whizz are good, you'll pay top money for a second hand bike from them, but you'll have backup for aftercare for a long time. Quick search on Facebook and the likes of this is...