Hi Graham,
Seems like you probably already have enough advice.
We have found nearly all pilots very happy with the Tenax bought from Avian.
You have the point about extra "Avian" chest buckles. Several pilots have been pleased to have these just after a zip failure.
Regarding sizing. The main point is to measure yourself accurately. I tend to go on about it but quite often we find pilots making personal allowances for whatever reason. The important thing to do is to measure in say jeans and T shirt. Do it twice to check you have it right.
Then let us know if you want a tight or loose fit.
My advice is probably different so some of the other here:
It does depend upon how much stuff you want to carry in the back pocket of the harness but I would say "Default Large" rather than tight or small. I think this is probably more important if you are a thin pilot rather than a larger pilot.
Other options.
Trimmer. (Worth having with the 'kick ass' system)
Parachute side: Normally recommend Left hand side. (So that it is on a different side to the VB rope. You can reach across with your right hand arm hopefully (Wrekage permmitting.))
Do let us know what sort of 'chute especially if you are using a dual parachute.
Drogue Chute. Don't bother. Can be very dangerous if used incorrectly.
Finally colours: It is really worth making very clear exactly where you want the colours if you have a none standard scheme. (You can't make it too clear.) Generally the front of the harness is much more likely to get dirty so perhaps bear that in mind.
I hope this is of some use.
Steve E