If you're compelled to config each game to use the Steam controller, I'm not sure why you wouldn't be as compelled to do the same thing with an Xbox controller other than by choice. They both only need as much configuring as you want if you want standard gamepad input, for as little as you care about that, everything the Steam controller would then offer you is additive.
Steam controller comes with native support like 360/One?
I don't know, the last time I read about it there was a plan to push community on creating config files for each titles or something like that.
If Steam controller comes with full support like 360/One, no problem (it still uses batteries, anyway).
If no, One controller is the best option.
Why even buy a PC at all?
You have to buy the parts, configure the OS, OC everything. I can just buy a PS4.
Do you know this statement is really dumb, arent'you?