What's the mileage? Is it geared right for what/where you're hauling? Is there any warranty left? Can you haggle a DPF service out of the dealership? Can you afford the financing? Do you have the right freight lined up to make your payments? Is it the right wheelbase for what you haul? Who was the previous owner? How detailed are the service records? What comes up in an oil filter crush test/oil analysis? Do you really need 500HP?