CX bike for a tall man - build or buy?

I have long fancied a cyclocross bike, and have been looking around for one lately after my Raleigh Record sprint got nicked a few weeks ago. My criteria are that it needs to have mudguard mounts, hydraulic disc brakes and preferably a bit above entry level groupset for about a grand. That alone is a pretty tall order, however there are a few bikes about:

Whyte Dorset  - has the TRP HyRd brakes and old Tiagra.

Pinnacle Arkose 3 2016 model which has full hydraulic 105

Planet X London Road with full hydro SRAM rival.

All the above are £999. At the moment Evans have a Genesis Croix de Fer with old Tiagra and HyRds for £720.

All of them look like decent bikes, however the problem is that I am 6'6" tall and none of those bikes come in a size larger than 60cm. My Raleigh was a 64cm and my Sunday best bike is a 64.5cm Canyon with a stack of 645mm. I am not the most flexible and have my Canyon set up to be pretty upright with about 35mm of spacers.

Alternatively I can get a Kinesis 5T disc frameset in a 63cm size for £500 and build it up myself. Back of an envelope calculation looks like I can get the new Tiagra groupset for £230 from Ribble, pair of HyRd brakes for £180, some fairly basic shimano or fulcrum wheels for £100-130 and would then be going over budget to get a few bits like tyres, handlebars and a stem that I don't have at home.

So my question is do you think my proposed build is a better move in that the frame would fit better, or should I save a few hundred quid and some time and go for one of the off the peg bikes?