servo tape would work. You can mount the esc anywhere te wires will reach. I also use a strong zip-tie, that way it's easy to swap / reposition. The xl-5 probably comes with a tamiya for the battery and cheap bullets for the motor.

But I have to ask.... why upgrade to an xl-5? Are you getting one for free? If you're going to buy the esc, why not save a little and get a slash.... which comes with an xl-5... and a new motor.... I've seen used ones for $100.

Or you could pony up for an entry level brushless system like the novak havok for $150 which is programmable and could later be moved to a better car.