Firstly, Congratulations
There is no rush to get everything done all in one go, for me part of the appeal is adding bit by bit and enjoying the difference the new thing makes (if that makes sense).
My usual order is something like this:
1. Power of some sort - BigPie fuse box is the easiest and most value for money way to go here. This opens up options for all sorts of stuff and makes life a lot easier.
2. Heated grips - If you've had them before you know, if not your missing out.
3. If you go touring, luggage - I prefer stuff like the trekker outbacks you mentioned but entirley personal preference.
4. Crash bars/bash plate - always a good idea.
5. Screen - Yes, the airflow works well with these bikes - especially in combination with the Halfords/Aprillia Tuono mirrors.
Hope that helps in some convoluted way. More importantly, get out and ride it!