Forget the ramps, they are a pain in the tookus. It doesn't take that much more to make it a tilt. Removable fenders, at least on the driver's side, are nice but really only gain you a couple inches of space to get a car door open or closed. Wood deck should be 2" (1 1/2 finished lumber) and I would go at least 2x8 or 2x10. It basically costs the same so wide boards are better than narrower. You don't really need treated and it really warps bad anyway. Just use regular lumber and pour a bunch of linseed oil thinned about half with paint thinner. On the rear where you have the flat bar or angle iron to hold the ends down, go ahead and cut a piece of plate to cover the whole back up to the first stringer. Otherwise every car you load bellys down and tears up that strap that's welded on to hold the boards. That always happens when you use ramps. As far as sides go, most car haulers don't have them, they are more for utility trailers but I have also hauled a lot of cars on trailers with the frame work sides as high as the fenders. It is just a different method of construction and makes them a little stoger if you don't want a real heavy frame underneath. For multi purpose it might be best, but consider that it will be really hard to ever load anything with a forklift, like if you want to buy a bunk of lumber.
In my old age I am of the thinking that I really need many trailers and one with the wheels under the deck would really be nice sometimes too. The trailer I use the most is a 12 foot single axle with the angle iron side frame. It is a dropped axle, nice and low and is what I haul lawnmowers ect but also gets used a lot for hay, iron, hauling the buggys for my sister's horsedrawn stuff when we go to parades. It is so much easier if I am just hauling a truck cab, engine ect.
I have one in the works that is going to be a steel decked tilt and I am going to bite the bullet and buy a winch for it too. I have been borrowing one from a friend and it has really spoiled me. It only takes minutes to load anything, whether it has wheels or not.
I have a set of travel trailer torsion axles to start with but hope to upgrade it to 7000# drop axles someday.
BTW, I didn't know Ford made a travel trailer, tell us more about that.
but I've seen lots of people put alot of time and money into a trailer house framed trailer that was almost worthless when it was done.. 
