As someone who electrified a GT LTS-3 i can only advise you that you must do it! E-bikes are totally misunderstood, they can actually help you get fitter than a manual bike, how's that for starters..?!

Your bike looks like a great contender. How to proceed really depends on what you want from it. If you want a friend there to help you on the hills and spur you on, then i think a little 250w hub motor in a spoked wheel is the way forward. You won't need a particularly long battery so a 'Fish' type should go under the main tube, and give a nice low centre of gravity, as long as it's 3in girth will allow the bike to fold. Second to that, put it on the top of the tube.

More power and it'll have to go on the top. You don't want it on the rack with that bike as the front will be too light. Unless you go for a mid-mounted motor. Mid-drive would be best for medium power (500-800w). The motor should sit under the folding hinge just nicely. Mid drive is more sporty as it uses the gears - but your gearing is very short, so you'll be wanting to try and squeeze the biggest chainring possible on there..

Me, i'd wanna go high power, like a 1.5kw set up. A big 48v circa 1.7ah battery should go on the main tube, but you won't lace a big motor in that tiny wheel - no problem, the big boys in China make spokeless 16in hub motor wheels. You can put 3,000 watts in there if you like!

With mid-drive, gearing is your problem. With hubs, the size of the wheel doesn't matter because the computer will calibrate the motor to the wheel - you may still want a bigger crank though, i can say from experience that very fast pedalling knackers you no matter how hard you're pushing...

edit...

Oh, something i forgot - you show your front axle as 100mm. Yes but your rear axle is 130mm. Some small rear motors (200-350w) are a bit wider, and this is no problem on a big-wheel bike as you can bend the frame a touch to squeeze the hub in. However, your 'Flex 16' rear fork is too small for this and won't flex enough... The big rear hubs (500-3,000w) are actually narrower and will fit straight in.

If you're considering a front wheel hub with good power, i'm inclined to dissuade you. Medium power may be okay on a 'lean forward' bike but a short, tall, upright bike like that will only be good for low power. This might just be ideal though, and with a hub on the front you can have a relatively light battery on a pannier rack above the rear wheel for good weight distribution.