Servos: Two main options to consider; standard voltage vs. High Voltage, and Coreless vs Brushless. Also consider whether you are OK with needing shims; MKS servos are awesome, but they may require shims for installation on the E5.
BEC: If you are powering the servos off a BEC, try to stay away from servos that are known to be inefficient.
Main Packs: Although the inner frame dimension is large enough to accommodate larger 5000mah and even larger packs, there is a weight penalty when using those larger packs. If you already have those larger packs, then use them. However, if you are in the market for new LiPo's, the smaller and lighter 4400-4500mah are worth considering.