If I was starting a new build I would seriously look here for a Groupset

http://www.chainreactioncycles.com/us/en/groupsets?f=2259

I know most of those are 11 speed though.

I would match your TT bike's drivetrain components, if you think you will be there for a while. To your point, it makes life a lot easier for swapping stuff (wheels mainly) between bikes in an emergency.

Spend time researching the crankset. The frame, crankset, and bottom bracket are a matched interdependent set. The crankset does not need to match the rest of the drivetrain. And, it is way easier to find a good (or good used) 10-speed crankset.

You can use anyone's brakes.

The rest of the stuff is pure personal preference, as long as they are compatible: frameset, seatpost, saddle, stem, wheels, handlebars.

First thing you need is a frame / fork. That will dictate much of the rest of the build (eg seatpost diameter, BB type, brake type, headset, stem length / rise)

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Swimming Workout of the Day:

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2020 National Masters Champion - M50-54 - 50m Butterfly

Get a torque wrench, too.

beer

always helps me do everything

A work stand and it's own area to work on it. Seriously, it makes a big difference in the experience.

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No coasting in running and no crying in baseball