I have purchased all 5 of the Horizon Rally cars (Celica ST185, Escort Cosworth, '05 Impreza, Evo VIII and 037) but for some reason the achievement hasn’t unlocked. Do I have to put the rally upgrade on all the cars or is this just some random bug?
Problem solved. It looks like I did have to add the rally kits to the cars. I changed the bumpers and as I quit back to the main menu the achievement popped.