Mik

As you go up the ladder in terms of features and expense, your returns diminish while your costs increase. Do you believe a 1Dx is three times as good as a 5Dmk3? Do you believe that a Ferrari is 10 times as good as a Honda Civic? I don't, but they'll sell less of them, to people who absolutely need (or absolutely want) the few improvements that it provides, and the a9 is the same. 
 
The a9 isn't competing against the X-T2, it's competing against the 1Dx, and the D5. 

I wouldn't get the 23, I'd buy a X100T for that. Get the f1.4 35mm and have done.

I'd buy an X_E2s or X-E2 (even a used one) over the XT2 or even the XT10 if I were a "newbie"
then you have money left over for lenses. 
 
As for "growing into a camera"..You could spend the next 5 years growing into an X-E1 or an X100s
and graduate to the XT5, by then you'll be tired of all the gear and carrying stuff around and use your phone  
 
After years of GAS (gear acquisition syndrome I sold my Canon's, Sony's,  Panasonics and all I have is the X-E2 which I LOVE, and a X100 that I am selling to finance a new lens. (classified here in classifieds).
 
Im a pro and on jobs I rent a Canon if I need one but get by with the X-E2 most of the time. 
 
The X-E2 with a 35mm f2 lens or the new 23mm f2 lens is also a small package you can carry around. you might not want to carry an XT1 or 2 around if that's what you do.  
 
X-E2 over XT10 because the viewfinder is larger and over on the side so you're not smashing your face/ nose on the LCD all the time. 
 
Good luck