What you buy is largely dependent on what you plan on doing with it, the term "Robot" can be somewhat vague. If you can give us more details on what kind of project you are interested in, I'm sure someone can provide some advice for you.
Are you interested in a remote controlled vehicle, or are you interested in something more capable with sensors, etc? If you are going with a remote controlled vehicle a transmitter, receiver, and servos would work for you. If you want any degree of autonomy, you would want a microcontroller. If you want a remote controlled autonomous vehicle (basically an autonomous robot that can also be controlled with a remote) you might want to add bluetooth, and use that instead of a transmitter/receiver pair. The more detail you provide the more helpful we can be.
For example, since it looks like from your part list you are looking more for a remote controlled vehicle some good information to provide:
What kind of design are you looking for?
How many powered wheels?
What type of steering (e.g. differential drive)?
What other capabilities (e.g. pan and tilt assembly)?
etc.
