i’m building up a decent spec road bike at the moment, so whilst not exactly the same my thoughts would be…
planet x: not sure what spec DA (is it the older 7800 or new 7900?) but take the cheapest you can get a groupset for, them what do you have left for frame, wheels, finishing kit etc and if you can’t get near it for similar spec then its a good deal? personally though i prefer the look of the ribble sportive.
used: open to argument, but i prefer buying in parts rather than complete as i think you can get a better deal ‘generally’. my build, which is a mix of used and new – very nice carbon frame, full dura ace 7800, wheels tbc but prob hope pro 3 on swiss rr 1.2 – i reckon will come in at a about 1.2k. new it would easily be 3.5k.
the other adavantage of the latter option is that when you come to sell on the depreciation is far less and in some instances if you are lucky in your shopping you can make a few quid.