For 2wd buggy, I would suggest to ask people at the track and see whats popular, as they will be able to help you if you have troubles. You don't need to buy new, but I am currently running a Team Associated B6.1 right now and it is a blast, real easy to put together and the kit setup is very easy to drive. I would recommend Hobbywing products for your esc and motor, as they tend to be cheaper than other brands but still have plenty of power. I don't have a specific motor suggestion but you can buy a Hobbywing Justock esc for about $50 online. It is cheap, but is more than enough esc for a 17.5t motor. For a servo, I would suggest the $60 Savox 1258tg. I have 3 of them and have never had a problem with them. They do like power however so you may need a BEC when paired with the Justock esc. For tires, similar to the car, ask the fast guys what they are running and why, I currently run Proline Primes (front and back) in clay compound on carpet, while applying traction compound between each round.

For transmitter and receiver, this depends on what you want and how much you want to spend. I started with a $50 fly-sky transmitter and quickly upgraded to the $320 Sanwa mt-44 that basically does the same thing as the fly-sky, I just wanted more options to tune and play with.

But, because you specifically stated you were on a budget, before buying anything I would see if anyone at the track has any used cars they are selling. Many guys have multiple cars, motors, and escs they are trying to sell for a good price.