FUEL

Essentially there are 2 approaches to getting this achievement.

The quick (ish) way.
If all you want is some (semi) easy G and you are not going for the full 1000 (this is not that quick really but quicker than the long way).

Go thru completing career and challenge races as this will earn you the "fuel" you need to buy new machines, you will unlock the ocassional purple vehicle truck (which you have to track down and collide with it (use a bike or atv during this phase to rack up the odd trick here and there as you go and work towards that achievement) to unlock the vehicle, also collect any barrels you encounter on your way to the truck for extra fuel) when doing the career races.

Use this "fuel" to buy the cheaper vehicles, and as more machines open up as you get thru the areas, the achivement will pop, but this approach may compromise your ability to complete the harder races as you won't have the fuel need to buy better spec machines.

The long(ish) way.
If you are going for the full 1000 in this game, then this will unlock in time. For me it unlocked as I unlocked the last area, this was basically as I decided that I only wanted to play the career races to start off with, (with the intention to do the challenege races in reverse order so they get easier as i go along) following that method I only brought the higher spec machines i needed to beat certain races.

This method is slower but you do get to open up more vehicles for progressing thru the career and earn the "free" purple truck machines as you go, whilst still leaving a fair bit of money/fuel in the bank to buy the very high spec machines you need towards the end (you will have more money if you do the challeneges as you go and not make life harder for yourself as I tend to do).

Some challenges gives you a vehicule, and you also can find some on the map. The best way to get money is challenges. It gives you about 2000 and then 4000 Fuel in the begining. If you are good enough play normal races in the hardest level, 5000 fuel for you.
Choose carrefully your buy anyway.
Don't forget that if you find some fuel on the road it gives you 100 credits ;-)