If you can fit one, i heard mid drive is the best, especially one comes with torque sensor. Pros: it seems easiest to fit and maintain, and from others that ride with it they said it's the best way to drive the bicycle. Con: it's the most expensive option.

The next choice is rear hub, but you will need a pedal assist sensor or torque sensor added to your bottom bracket to get the function. Pros: it drive from the rear, pushing all the weight in front of it in a much stable way. Also cheaper than mid drive. Cons: it will makes servicing the rear wheel much more complicated with all those wire running around.

The last choice is front hub, it's for when your bike's rear hub spacing can't fit a motor hub and you can't reasonably cold set it. Not an issue for more recent bike, but if you have old bicycle with single gear, this is always the case, like my old bmx. Also for when you have internal gear hub. Pros: it's easier compared to rear hub to convert your bike, and maintenance is less complicated. Cons: it will skid a lot when the motor kicks in. Like, a lot. Especially when you throttle to help start the motion. It basically trying to pull the weight of the bike, which usually at the rear, and it will have some hard time gripping the ground while pulling the load, especially when you goes full throttle. In my experience, it's not possible to ride in full speed the motor is capable of because of the fluctuating traction. It's also made for a bad sand bike as the front will very likely dig into the sand and make you fall down(experienced this myself). Still, it's very capable to ride up to 23kmph and pull me uphill, which is good.

For the motor power, it based on how hilly your ride is and how heavy the whole bike + cargo + rider is. In general 250w nominal 400w peak is enough to handle anything, anything above that will give you a bit of boost if you're on a heavy side. Mine is front hub 36v 250w nominal 450~500w peak, and it can drag a roughly 95kg total load(55kg me + 35kg bike + 6kg grocery) to go up to 23kmph, albeit i need to pedal harder when going uphill. Though for the voltage, 36v is the minimum you should have, 48v is much more common and can deliver the power better.

As for the battery capacity, since you mentioned intercity, i'd suggest something more than 10ah so you can commute 2 way without charging. 15ah is ideal, taking those range anxiety away. My 15ah lead acid battery(i know i know) can go for 44km round trip with 60% or so capacity to spare, with climb around 220m.