More people will have better info than I, but Im fairly new to it too.

Batteries. I use the stock one and a 2100mAh one of the same brand, both using the stock charger for now. Unless I have a problem with it, Im not going to buy another charger.

For the servos, The only one I've replaced is the tail servo to a "Spektrum 7.5 Gram Digital Programmable Super Sub-Micro Servo." It can be set up in digital mode, as opposed to the stock one in analog only. The other 3 servos have not given me any problem.

Maybe I lucked out with my batteries and other servos, but Im the type of person who will keep using until they give me trouble before upgrading them. Im learning on my B400 and replacements for stock parts are dirt cheap around here.