All great advice, the one thing that made a huge difference for me was the geared tripod head as Mt Spokane mentioned. I started with a Manfrotto 405, then went to a Sunwayphoto GH-Pro, then moved to my current Arca Swiss D4, the top end architectural photographers head is the Arca Swiss Cube but is really primarily for heavier camera setups than the normal 135 format digital camera.
I'd highly recommend getting the best geared head you can afford, for architectural shooting it is the number one aid and I'd value it above a tilt shift lens. I'd rather go out with my 11-24 and D4 than my TS-E17 and regular tripod head.
As for gear setup, if I was starting today I'd get the TS-E17 and EOS-R with the filter holding adapter first. Then I'd get the 1.4TC followed by the 2xTC, next big expense would be the TS-E 50. Third lens would be the 11-24. An iPad is close to essential too, well once you have used one it is difficult to go back to not using one.