My advice, buy a truck! Seriously from reading your other threads and having an idea of what you equipped with, the first thing you need over a new mower is a truck.
Other than that, I couldnt find your cars towing capability. I'd go with the smallest trailer I could get and still fit everything on it. If I remeber you just have a push mower so I see no reason to get more than a small 5x8, and a mesh floor would make it even lighter. Im not basing this recommendation on what is best for your business, but on what is best for your car. I think if you buy a trailer with a 3,500lb axle and start putting 2-3 yards of mulch on it your car will not last.
I wont recommend this to you, because Id hate for you to do this and then the lawn care thing not work out for you. If it were me, I'd sell my car and get a 1/2 ton truck, atleast a little Ford Ranger or Toyota Tacoma. Trucks are typically easier to get loans for than mowers. I realize that is a lot of commitment, but the opportunities that opens up are wide open. You can upgrade eventually to a 16ft tailer and care 7 or 8 yards of mulch at once. You can do leaf removal, haul plants/debri, etc.
You may be able to trade your car in and/or get a cosigner. I know you said you need your car for a current job. I would be better to keep your current job and make do than buy a truck, but if a truck works for both Id think about it.
Find out the towing capacity and tongue capacity (this is usually on your hitch or bumper somewhere) of your car and trailer hitch, if you can safely haul atleast 3,500lbs I'd change my advice a bit.