Re:
I recently bought a full carbon 29er which was an absolute bargain imho.
It's a felt 9 elite, and whilst I wasn't shopping for a new bike, I stumbled across it, and for the price thought why the hell not. Rrp was £2.5k for a bike kitted out with sram x-7/9, Reba forks etc.
I started the process of upgrading, selling off the x-7/9, avid brakes, Reba forks, wheels etc and replaced them with x-0, Sid, FSA carbon cranks, easton carbon bars and post, eventually wheels too.
I shopped around, took advantage of sales, bought and serviced parts where needed/could not afford new prices and ended up with a bike that would probably retail at the £5k mark for about a tenth of that.
I guess I'm saying if you are in no rush, hone your final build plans and budget.
If you have the time and inclination, buy a bike with some bits you want, use as is, and gradually source the dream build parts when they come up at a price you are willing to pay.
Whilst it might not be your dream bike, I ended up with this. It's had me out riding more in the last few months than I've ridden over the last few years.
I still hang on to a couple of retro garage queens (more than I should), but I ride this for hours on end and it's really comfy, light and aside from going crazy on a wheel upgrade (which I'm not doing), it's about as high spec as I could want (I blame retro bike for turning me into a parts snob).
I do have a nice build I've barely ridden. 26er macmahone the sword frame, Reba bolt through forks, xt 10x3 drive train (mostly new), ritchey wcs wheels and some nice finishing kit I've ridden once since building. I've pinched the brakes but can add others. I should sell it really....