To get this achievement you need to buy EVERYTHING from the Motor Forge... APART FROM:
Mt. Rockmore Statues
(These two things don't count.)
If you need more Fire Tributes (aka money) to buy stuff, then do side-quests, hunter missions, etc.
If you've done all those, and you need more money, just ride around until you find some of your team that are under attack by the Tainted Coil. Help them kill them and you'll be rewarded.