>>3815772Xt2 is definitely more pro friendly than xpro2. Bigger viewfinder and battery grip option.
If you want to have lens, 18 f2 would not be my first thought. It is pancake lens with some compromises to accommodate this bright f2 aperture. Honestly, go for 18-55 f2.8-4. Despite it being kit lens, it has great build quality, very good image quality and is still very small and affordable. On second hand market it is cheaper than 18 f2.
Idk about flashes. I use godox, they are best bang for buck option.
Pic related is iso 12800. A bit rough but imo still workable. And on a last though you may want to rent it out to check AF performance. Idk how important it is for your work, but I don't think xt2 has as good af performance as d750
