Upgrading Cube Aim 29 2013 I'm thinking about upgrading my bike Cube Aim 29 2013.
Current configuration described here.
The bike is completely ok for short rides, but for longer rides doesn't feel comfortable. So I'm going to change saddle and pedals, but I also want to make it more suitable for off-road rides.

I'm considering Shimano components. Is it reasonable to buy something like XT M785 10 Speed Double Groupset? Or SLX level components would be enough?

Regarding the frame - it is in good condition and Cube uses the same frame for their higher-end hard-tail bikes.
Don't buy that XT groupset for that bike to solve those problems - it wont work.

XT is great, but it won't make your  bike more comfortable, and it wont make it better off-road. Worse, it wont work for that bike since it has V-brakes, and that groupset has disk brakes.  They are not compatible.

To make it better off-road you need (in this order) better forks, disk brakes,  better components (lighter and stronger) and lighter frame.

I agree with what others have said here: sell it, and upgrade the whole bike.  If you really can't do that, only go for SLX.