If I understand you right, the emphasis is on off-road ability, The most expensive and vulnerable part of your kit will be the battery, which would be likely to get a bit of a shaking. Mounting it in a bag might work as long as you don't get one with the small 18650 cells. Pouch-type cells should be more robust as they have less connections. Mounting the batteryon the seatpost will need a fairly strong or well-braced rack. The aluminium ones aren't strong enough. You can get steel ones very cheaply on Ebay, which is what I use for my 7kg Ping plus toolkit etc, but they come from China. You can use an aluminium one as long as you add a couple of extra struts to a lower position on the frame or seatpost to stop it flrexing at the joint. The Frog type batteries have fairly strong steel mounts that also house the controller. You need to check if there's enough clearance above your backwheel at maximum compression.
If you're going to do serious off-road (jumping etc) then you need to think a bit more about the motor. There's only one crank drive kit that might be suitable, but I can't remember who makes it, pehaps Flecc can give a link. It had a name like Sunstar IIRC, but I don't know how easy they are to get hold of. A guy on the forum was looking at one, but never reported back. That leaves hub-motors. Small nylon gears don't go well with jumping, but on the other hand they're cheap enough to repair and even if you don't want to dismantle one the whole motor only costs about £100 to replace. Bigger ones like Ezee and BPM will be stronger but heavier. Tongzin is too fragile. Nearly all the americans use larger direct drive motors for off road which need a lot of power to climb hills. There's one small direct drive motor that I've seen that might work, supplied by Golden Motor and others, but you have to get it from China and you have the problem with drag.
If you want to buy a kit that just bolts on, your choice comes down to a Juicy kit with Frog battery and hope that the motor gears last long enough, or an Ezee kit that has a much stronger and more powerful but heavier motor. They supply a bag for the battery that I think can be clipped to the cross-tube, but if not couls easily be adapted. The Bionx kit might work, but problems are very expensive to repair. They're relatively heavy too.
Whatever you choose, it'll be a compromise. Everything has advantages and disadvantages. Ther's no perfect solution so you won't find it.
