Forza Horizon 2

The cheapest car you can buy is the VW Rabbit GTI which costs 5000CR. You can buy cars at the Autoshow, just hit X then choose Value, and the first car will be the Rabbit. You'll be able to buy cars after completing the road trip to Nice.

Also if you've bought the Day One Edition, you can purchase the cars from the free launch pack for 0CR. Thanks ScuzzyBunny for this info.

Complete the first championship and showdown. After this, you will be taken on the road trip to Nice. Upon arriving in Nice, you should have access to the Autoshow. If you bought the game new you should have received a code to access 3 cars for free, a Fiesta, a Mercedes Hot Hatch and a Challenger. Activate your code and download the pack, then purchase a car for 0CR. There is also a 5000CR Volkswagen you can purchase to unlock the achievement if you do not have the Day One edition.

I personally got this by downloading the "Launch Bonus Car Park" for free from the Store before the game had even released (Free DLC? Why not!) the DLC includes 8 Cars to choose from. You can buy any of the cars for 0CR from the event HUBs (garages) in Horizon by going to the autoshow menu.
THE FOLLOWING LIST ARE THE CARS INCLUDED:

2012 Cadillac Escalade ESV
1968 Abarth 595 esseesse
1956 Ford F-100
2003 Ford Focus RS
1997 Land Rover Defender 90
1987 Buick Regal GNX
1993 Renault Clio Williams
2015 Ford Mustang GT

Alternatively if unlike me (rented) you bought the game you should have access to the DAY ONE CAR PACK by redeeming your code inside your new copy of forza horizon 2. As with the above method these can be bought from the Garage Hub for 0CR. The Pack Includes:

2012 Dodge Charger R/T
2014 Ford Fiesta ST
2013 Mercedes Benz A45 AMG

If I'm missing anything from this guide please let me know!

**EDIT**
The reason why most run into a marketplace error is that they are selecting a dlc or pack car. Select any car without the "down arrow over a bracket" symbol, and it will allow you to purchase.

This is a tip to people starting the game after the servers have been shut. I didn't have an option to buy a car, that I could select without getting a marketplace error, until I had finished the second championship (9 events in).

Once this had been completed,I was instructed in game, to go to a car meet. From there I could select from one of the other cars and I purchased one.

Hope this helps people as I had no luck purchasing a car from the Auto show.