I tried a similar build half a year ago, also based on arbalest, quarterstaff and light armor
I worry that the quarter staff sticking up off my back will get in the way of my xbow aim.
yes, it will get in the way and it´s annoying. Great to hear that you could handle it, I couldn´t :I
Imo you should make yourself a skip the fun character and try it out for a longer time, so you can see how the build works out in any situation you might face
But let me warn you:
you will have a damn hard time with shielders. The only hope in an encounter with a shielder is that he/she makes mistakes. You won´t be able to break their shields with the staff and if you can live with that, you´ll be fine. I couldn´t ;I
I would stick to 7 WM and 7 ATH for a stf build and spend a bit more points into power strike. Otherwise it will also take you ages to kill a simple guy (and shielders will bugger you even more)
9 ath is great for "hit and run", but you can´t run away all the time (cav, other ranged); it may get annoying
150 wpf for a normal xbow is fine! I´d even go for 15 str so I could use a heavy xbow (if you wanna do that, you can even go down to 145 or 140 wpf, because the heavy xbow its very accurate), since you´ll still be quite fast with 7 ath (and with lvl 33, 8 ath )
in a nutshell: try it out on a stf build. Play on EU1, play on EU2 and play it for a couple of weeks. If you then still have fun playing it, then you´ll also have found out what build you want. Then you´ll be ready for an alt and a lovely lvl33 endbuild :>