For a nitro buggy, you need two of course.

The required specifications for buggy STEERING servos are basically, 170oz-in or more of torque and as fast as you can afford. Ideally you want over 200oz-in of torque or more, but 170 will do if on a budget. For the THROTTLE servo, at least 100oz-in of torque and again as fast as you can afford down to about .12s/60* any faster is of no benefit.

What makes servos good and bad is their materials, weight, speed, torque, current usage and of course reliability. Good brands are Ace (now branded as Team Associated XP servos), Hitec, Futaba, Savox, KO Propo, Airtronics (aka Sanwa), and others but they are the main ones.

My budget picks...

Steering: http://www.amainhobbies.com/product_...tal-Gear-Servo
Throttle: http://www.amainhobbies.com/product_...-Digital-Servo

Medium budget (I use both these servos in truggies and buggies, they've never let me down, fast and strong)...

Steering: http://www.amainhobbies.com/product_...rvo-w-Heatsink
Throttle: http://www.amainhobbies.com/product_...rvo-w-Heatsink

Money no object:

Steering: http://www.amainhobbies.com/product_...-Digital-Servo
Throttle: http://www.amainhobbies.com/product_...rvo-w-Heatsink