Yes, it's that simple but in reality it is not. The discs are extremely rare but I did see Zimmermanns on ebay/runautoparts recently.
If you can't source discs locally then your next bet is importing them from Europe as there are several aftermarket choices available.
It makes a 135i hub and bearing swap seem far more straightforward as all the parts can be sourced locally (even if you have to buy knuckles from a wrecker to harvest the hubs). Wheel bearings are easy to get.
The handbrake should be fine as 130i uses the large handbrake.
The only other option that I considered is the 330i/335i rear brakes. Easy to find calipers, carriers and discs. Pads have tons of choice. I have F30 335i calipers on my front axle. I have the F30 335i rear calipers handy also but it takes some custom work to make them fit for several reasons. Waiting on some scrap F30 discs from work to mock it up.
Send me a PM when you've sorted out some discs
