I think I see where you are coming from now, from your other threads plus this one, but do heed what ClarkSr and Mo Predator Racer have said above.
That said, you have 2 limiting factors - the 2006 Mustang is definitely a limiting factor for max trailer weight; not the best choice for towing, but it can work if you heed the advice on structure by Ted Hamilton in the thread you started on the Harbor Freight trailer. To add to what ClarkSr said about a shoe string budget, remember this; in spite of how some people view karting, there is NO cheap way to go racing, only some types of racing that are less expensive than others. This is an unfortunate fact. If it means you have to delay your start in racing a few months in order to do it with adequate equipment, that is unfortunate, but, in the long haul, the best decision. Best of luck, and don't give up.