We've used rs550s, different brand, for First Robotics Competition. Basically they are inexpensive drill motors, typically geared down. Don't last if stalled or overheated. Don't block those front vents. As I recall motor built-in fan pull air in back, and out those front vents(by shaft)
Brushed motors with plain bearings will likely not last as long as brushless/ ball bearings? I don't know in your application. Weigh the cost against machining & time. At 18,000 rpm, it needs to be good. You're saving $10 on motor, and $20-30 on ESC?
There are people using brushless hobby motors for other things, like milling/routing. You may want to research a few forums and see how they hold up:
Can RC Brushless Motors be used as Spindle Motor ???
And what the hell, if it doesn't work for the A/C, duct it into the intake and market next generation E-Ram!
eRAM electric supercharger ( a brief description) & some responses to the many post