Running a brushless DC motor on low rpm using servo tester
For 15 RPM you want as many poles as possible and sensored. More poles results in smoother operation at low RPM, and sensorless is only reliable for applications with zero startup torque and high running speeds. Even with sensored, I suspect you may have issues with radio controlled equipment since 15 RPM is pretty damn slow. I would run it by some RC truck guys first to see if they think smooth reliable operation is possible at 15RPM. The airplane guys won't be of help since they don't use sensored since it's more expensive and propellers are low starting torque, high RPM applications.

More poles can limit speed at higher RPM since the ESC must commutate much more frequently and faster per RPM, but it shouldn't be an issue at 5000RPM.

You may have resolution issues with a servo tester since the knob will be able to go from 0 to 100% with the maximum RPM probably being in the thousands of RPM. I'm not even sure if a RC ESC has enough pulse width input resolution to reliably read and operate 15 RPM.

I would consider gearing to better accommodate the low RPM side but that makes cost spike considerably. You are probably going to run into torque issues at 15RPM without gearing even if it does rotate smoothly and reliably.