I'm thinking along these lines for the most part, although my logic is slightly different... You can't get much better of a bike for $10,000 than you could for $5,000. The other $5000 you can spent on a $10,000 bike is basically money thrown away. I'd be looking at carbon Cervelo's, BMC's, and Look's. I'd also be looking at titanium Litespeed's or other well respected Titanium bike manufacturers, which would be better for dependability. You don't need custom. You can get any bike of the right size to fit perfect. Going custom would be another waste of money, AND TIME, in my opinion.

Edit: Look at this thread. Cervelo, BMC, Pinarello, and Look indeed come ahead in people's mind for good reason, they are spectacular bikes made by companies who put the most thought into their design. Start with full Campagnolo Record and Zipp 303's and you'll have a dream machine that you wouldn't be able to make much better, except in the details such as handlebar, stem, seat/seatpost, etc. which you could go full "weight weenie" with and THEN indeed have THE BEST BIKE, since that's what you're looking for after all.