A 5.5 is too fast for a buggy. Even pros don't usually use less than a 7.5. Most people use a 7.5-10.5 in mod buggy.

The saddle is the same size as a stick pack, and is a good choice for the b4.2

As for an esc, I like hobby wing escs. The v3.1 is their main esc to use for mod applications.
And for a servo savox offers good performance at great prices. Futaba and hitec are the other main option if you want to spend a bit more.

Just look for at least 120 oz in of torque and faster than .12 speed.