Hi - Yes I agree using second hand parts carries a risk but by very careful search and selection you can find virtually new, pristine parts for a fraction of the price. For example the Stronglight Twister Ti ISIS BB was €160 from a bike shop but found a brand new one, never fitted, for sale on Troc Velo for €26. Asked him why so cheap and he said because it was italian thread and it was hard to sell. Carbon pro cranks - completely mint condition for £70, again not high demand because not many road bikes running ISIS hence fewer bidders on ebay. Pro-lite wheels £140 with only 500km on them, absolutely mint condition, hardly run in yet. Sold because they were Campag and the seller had switched to Shimano, Less people running Campag. Centaur carbon shifters and rear mec bought off a friend, totally meticulous by nature, as new condition €80.
In most cases I've then sold the old pieces for nearly as much as I paid for the upgrade parts. So, by choosing lesser known but quality parts or less common bike specs you can keep the price way down. Final example Xpedo Thrust ti pedals - hardly known in cycling circles, lighter than Look Keo Carbon ti blades, made by Wellgo so solid reputation for quality, Trying hard to break the stranglehold of Look, Time etc on the road pedal market. Used for 2 rides only, mint condition but bought for half the list price including a second set of Look keo classic pedals, which bizarrely also limits the amount of bidders.
I've worked out that to buy all the parts new would cost over €3600 so to me €590 is well worth it of such a high spec bike.