Looking for advice please.

[figbat]

650b would be a wider rim, capable of running wider tyres, possibly more comfort at the expense of speed.

650b is a smaller diameter. If running 650b wheels in place of 700c you can often fit wider tyres but this depends on fork and frame clearance and that the frame is set up for either.

700c wheels will generally give a better ride over rougher ground. Plus there are more tyre (and wheel) choices.

 

[MichaelW2]

Normal flat bars put your grip at the wrong angle. A bar with some sweep ( like On One Mary) relieves some of the rotational stress in your arms.
As discussed, rack and 'guard mounting is essential but avoid propriatory mounts, go for standard M5 threaded eyelets.
Disk brakes are good, but to play nice with normal rear racks, they should be located on the chain stay not the seat stay.