Has your model been stored in a place with a fairly constant temperature? Most covering will last indefinitely under the right conditions. If it has been exposed to high heat as in an attic or garage ceiling or conversely in the cold of a garage it might be well to do a punch test as used on full scale aircraft. I-26 driver is right abut the balance point. Most models are designed with the main spar as a balance point because it is usually the longest part of the span. As to a 600 mAh battery that is fine if the voltage is correct. I use a 2400 mAh in my sailplanes and pay the penalty in weight in exchange for longer run times.