It's been said, and I believe it's true, that if you have a nice bike, don't ruin it with a motor. I've put a number of motors on bikes, but for me, it's usually about making a bike that no one wants to ride into a bike that's fun with electric power. If that's a nice bike, then leave it alone.

Here in the USA, I can buy a cheap chinese import folder with the battery hidden inside the frame for less money than it cost me to buy a motor and battery separately. The problem with folders is there's no room to mount a big battery and keep them folding. You need to look at them as shorter range vehicles. Being folders, you don't go very fast anyway. I do have a 20" folder converted to electric. Tried to keep it as a nice bike, and I like it, but an hour of riding is about all I can do unless I carry extra batteries in saddlebags.

Check on UK Electric Bikes l Pedelecs for advice on small hub motors. They have a lot of users that do conversions and order from European and Chinese suppliers.