Dirt Rally

This is an easy achievement to get.
In the main menu go to 'Career' and then 'Service Area' in the middle of the bottom row.
To unlock the first engineer slot you have to drive a total of 46 miles (74 km). This is cumulative and you should get that in the first events.
Click on the first empty engineer slot right next to your crew chief. Then choose one of the many engineers in the list to hire him for some credits. After you hired him, press on the engineer to buy and equip a bonus. Again, just choose one of the many bonuses in the list to buy for some credits.
After a few seconds the achievement should pop.

If you want to save your credits as much as possible, I think the cheapest way to get this costs you a total of 3700 credits. 3200 for the cheapest engineer and 500 for the cheapest bonus.

Edit: If you vote negativ on this solution, please let me know in the comments what I could improve