For this achievement you need to purchase every upgrade for any car, I did this in story considering everything is cheaper in the story and is easier to get money. Anyway go to any of the L.A. customs and use any car I picked Michael's car after jimmy stole it and upgraded it and do finished all the upgrades. I bought everything in each category just to make sure the achievement popped. GOOD LUCK TO ALL MESSAGE FOR HELP!!!
If people are still having issues try reading thru the comments and see if anything works for you! Everyone is leaving good tips!