Sounds like a good plan!
The Teflon insulation isn't really necessary; it's just what I used but the inner from RG-58 or the like would have been as suitable. I've used RG-58 inner on other couplers that I've built.
The brass tube is grounded at one end only
; the left-hand end in the photo.
This forms a Faraday shield which blocks the electric field from the through line but does not impede the magnetic field.
This prevents unwanted effects from capacitance between the transformer primary and secondary.
If the tube were grounded at each end it would form a turn through the core.
You will see similar shields on power transformers; a copper sheet between primary and secondary/ies.
It has a gap in it for the same reason; to prevent it being a "shorted turn".
I said "unwanted" above but this effect is not always unwanted; the combination of both electric & magnetic fields is the basis of such couplers as the Bird-style.
You may wish to omit the shield as it would interfere with the "tap" for the other transformer; this tap, ideally, would be in the centre of the through-line inside the current-sensing transformer but this is difficult, although not impossible, to achieve in practice and as close as is convenient will work fine at HF.
Omitting the shield will not cause any problems; I built the W7EL meter years ago with the microstripline described in the article and it worked fine.
Teflon has lower dielectric loss than most other plastics so it is more "transparent" to RF fields; this doesn't really matter in non-critical applications but it begins to matter in microwaves.
Almost any size of toroid would do; a 37 would be fiddly and a 50 would be easier to handle.
This is an excellent toroid calculator that I use a great deal;http://www.dl5swb.de/html/mini_ring_core_calculator.htm
You can use it to find the length of wire needed and what gauge wire would fit.
The 0.25mm wire I used on that coupler fits nicely on a 50-size toroid; it gives about 30 degrees of spacing between the ends of the winding.
Winding the coil around about 330 degrees of the toroid, as distinct from winding on a smaller part or on the whole of the circumference, has been found (by W1FB initially) to minimise stray C and is fairly general practice.
The toroid, ideally, will dissipate no energy; it serves to couple flux between the two windings and, since the flux is equal but opposite in each winding, there is, in a perfect world, no net energy dissipated.
But the world is far from perfect; the core will
dissipate some energy. Although it's not as simple as you describe. experience has shown that a 50-size core can be used in wattmeters of the power class you are looking-at.
The 1/4-watt is what will be dissipated in the secondary load resistors; you will often see 1/2-watt or more resistors used in that location and that is what you should use in your design.
I found an interesting paper in my extensive collection (I'm a voracious collector of useful information
); back-tracking via Google found the source;http://lcbsystems.com/bridgelevels.html
The figures are for a 10-turn coupler but you can re-work them for your design.
EDIT; 'BYU obviously types with more than one finger.