Yup, a UBEC is simply a device with two wires in (goes to the main battery pack) and two wires out (goes to the ESC). Because most speed controllers have a built-in BEC, you have to disable it when using an external one. Some ESCs, like the Warriors, have a BEC disable jumper (black jumper). Others you hyave to cut ther red wire in the throttle cable. Or, just use a short servo extension with the red wire removed so you don't have to modify the ESC throttle cable.
You can get a UBEC
here. It's a little on the large side, but works well. You can get a cheaper one
here, but the support and customer service at RC-Monster is second to none, so I try to get everything I can here.
As far as pinions go, I's get a 14T and a 15T so you have a few options. Also get the three various Revo spurs (36T, 38T, and 40T) and you should be able to find the sweet spot between speed, runtime, and ideal temperature with one of those combos.
If you go with the Wanderer 8XL, you should also get
this heatsink and clamp. The Wanderer end-bell tends to pop off from jump impacts and this heatsink also clamps it in place. Since there are none of those motors in stock, you could get a
Feigao instead. It has a built-in heatsink and the end bell can be removed for bearing maintenance should it need it. Since the heatsink is built-in, it is cheaper. FYI: the Wanderer and Feigao have the same KV ratings so the Feigao 8XL will be a good choice.
AFAIK, the Wanderer and Feigaos both have a 5mm shaft, so you should get one of the "5mm bore, mod1 pitch" pinions on
this page. Maybe someone here can verify that the Feigao really does have a 5mm shaft, but I think it does.
As far as ESCs go, the Mamba Max is a solid little performer. It is smaller, much more programmable (via a computer), can run brushed or brushless motors (with configuration changes), is easier to mount, has a built-in low-voltage-cutoff should you decide to go with lipos in the future, and is cheaper. I would definitely get a small 40-50mm fan to cool the heatsink though.