No parts missing.
Battery replacement isn't hard. Remove the 4 bolts which attach the forward chassis and you can access the battery compartment with ease.
Instructions easily followed IF you have encountered Meccano before. I would not recommend this as your first kit.
Certainly if you are not paying attention you will do things which result in problems. The drivetrain and supporting structure in particular needs to be carefully aligned so that it does not jam.
The kits primary focus is to make a car which looks good and moves. It achieves this. However there are too many plastic components and friction joins to say that this is robust. The motor is quite underpowered for the weight so a crawl is all you get.
I think Spin Master need to refocus Meccano back into mostly metal so that they can make robust models. Reading reviews this is what people want. If we wanted plastic we'd buy Lego.