What capacitor(s) will be best replacement
I needed a 6 uF replacement and only found a used 5 uF and a used 1.25 uF. With the help of several answers, I decided to only install the 5 uF capacitor.

The motor started fine on its own. Having assembled the pump with its pumping "blades" (extra mass) and the outer case it still started fine in my workshop.

Then I re-installed the 1-inch hose and the supporting rope and tested inside our gray-water tank. I learned that I now have less starting torque. But in the wild, the pump is starting just fine, even suspended almost 1 meter down in gray water, i.e. having an actual "load" in the outgoing hose. It is not mud, just water from our washing machine and shower.

So, nominally, I am 17% low on capacitance, and the used capa might even be somewhat lower than what its label says. But my pump is repaired and we no longer need to run with buckets to flush our toilet.Thank you all. Maybe this will help another user...

Update2: It is now May 18th and the pump is still starting every single time with load (in the water) at first flip of the switch. It never hesitated with my replacment capacitor with 5 uF. In the meantime a brand-new 6 uF capacitor has arrived in the mail; but I do not even want to open the pump again. I will keep it as a spare part for when my second-hand fix will die of the heat some day.

Update3: Today Oct 11, 2017; pump still starting fine every time. And keeping the replacement capacitor on standby.