In a lot of electric conversions people use permanent magnet synchronous (electric) motors. So that is where I started. Well known motors are the Golden Motors. They are fairly cheap, available in a range around 5 kW, and fairly easy to control (read, controllers are available).
But all those engines have one big disadvantage, to be able to use the maximum output (e.g. 5 kW), you need a very high rpm, up to 3500 or even 5000 rpm. Because the output of these engines is torque x rps * 2 * pi, and the torque is determined by the current, and that current is limited by the controller, so the output is limited. Also when the output voltage drops of such controllers, the heat losses increase. And of course you need a reduction to convert those high rpms to something around 1000 rpm to adapt to the propeller. This reduction, the losses and the fact that the prop need to fit very precisely in terms of diameter and pitch makes this a less favourable motor for me.
From Daniel Boekel I got the advice to look for a motor from a fork lift, these motors are serial motors, which have a much better torque / rpm relation. The maximum power is already achieved at low rpms.
With this in mind I started searching on marktplaats (the Dutch equivalent of eBay), and already after a couple of days, a batch of 5 motors caught my attention. They were 10 kW, and were used to drive a hydraulic pump. Oh, and they were heavy… (approx. 70-80 kg). And only EUR 200 each, but they were sold as a batch of 5. So I contacted Daniel to ask if these were alright, and if so, he could have 4, but I wanted number 5 :-). He agreed and I collected number 5 from Alkmaar. So it finally had started, time to prepare all the other steps.