What makes a good donor bike ? Not sure it is quite the same criteria as choosing an analog bike ? More emphasis on how rugged the components are rather than lightness, particularly the chain / gears in a mid drive that is going to have to cope with several times the power a human could generate ? Hydraulic brakes. Also gears, don't really use the lower gears on an electric bike
It depends what you want to do with the bike. For basic transport a triangle framed catalogue bike with disc brakes and 1 1/8" steerer suspension forks works extremely well. For longer commutes, say more than 10 miles each way, a bike with air suspension at both ends becomes increasingly more desirable. These take some beating for that and they can be had from about £250:
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If you're already a keen cyclist or live in an upstairs flat, then a lightwight hybrid type with rigid forks is nice, but you need a lightweight kit to go with it, otherwise it's wasted and you only get the disadvantage of a rougher ride.
As you say, when it comes to crank motors, you're much better off sticking with older bikes that have 9-speed cassettes. 10, 11, 12 speed offers no advantage when you have a motor. They just cost more when they break or wear out, which happens faster than with the more rugged 9-speed ones. Even 8- speed would be OK.
A decision shouldn't hang on whether it has hydraulic brakes or not. If you can find a cheap enough bike with cable discs, it's very cheap, quick and easy to upgrade them to hydraulics.
If you want a front motor for whatever reason, you should look for a bike with steel forks and sizeable drop-outs. The rest doesn't really matter. If you're the type of guy that chooses a front motor, you probably don't know or care too much about bikes, anyway. Still, it should have disc brakes and a suitable place to put the battery. Definitely don't choose a bike with expensive suspension forks because the motor will stop the forks from working properly and they rarely have strong drop-outs.