Brushless motors require specific brushless ESCs. Make sure your ESC is built to handle a brushless motor, or just get the EZrun combo motor/ESC. The 5.5T can be pretty fast, specially if powered by a 2S pack. My brother has that setup, and he's had his share of "high speed crashes".
The use of LiPO packs require a low voltage cutoff ( if you want to keep your packs healthy). If your current ESC doesn't have that feature - an external LVC unit can be placed in line to protect the battery pack. I set my LVC @ no less than 3.3V per cell (for my 2S packs my ESC shuts down @ 6.6V).