Welcome to the forum, nice bike~! seems a shame to weigh it down with even more kit
if you are noob then a Hub motor is your best option and the parts that you have suggested so far are all fine, the X5 is very heavy you could look at the new HT motors as they are lighter however as they are so new not sure on their durability yet, although my buddy deecannio on this forum is about to test this out for us on his bike (we are both in the UK).
Yes Lipo packs are fine, 50V 10AH is pretty small, it depends on how long you want to stay out and how hard you ride also how heavy the bike rider combo is, to be honest the X5 wont really light up on that bike until you give it at least 72V 40A, I would be looking at 15AH for decent ride times, the frame makes it a good candidate to stash cells, get them nice and low in the frame, CA is a good idea and pretty much essential as you are a noob, charging can be dealt with by using a multi channel hobby charger or PSU and balancers, I think the Hobby Charger route is a good way to go.
There is more than enough info on the forum for you to make a very decent bike, you will also need to factor in torque arms for the rear axle as well, you may have to fabricate something as the off the shelf ones will most likely not fit and or handle the power esp off road.
Get this right and you will have one amazing machine for sure
I love riding off even on my normal road steel bone shaker! check some of my videos and posts and you will see what I run on my bike, I also have a little video showing how I charge my batteries on my youtube channel, I will subscribe to this thread to see how you get on.
All the following my vids from my youtube channel, subscribe to it if you like for more
My last video with my normal bike riding fast off road
My charger set-up, I can charge quicker now in under an hour
What you can do if you really want to scare yourself, thats deec with the bloody arm!