Pony Express gave you some great tips, I wouldn't buy anything other than a dump trailer with barn doors. The ramps will be needed at some point, thats almost guaranteed.
I personally prefer the low profile dumps, easier to load and unload. Some guy's wouldn't care about having a single or dual ram, but from experience I prefer dual hydraulic rams. If at some point it's an option and you are able to, upgrading the reservoir size helps when a sneaky or not so noticeable leak develops, you don't want to run your pump without fluid in it...Having more fluid may buy you some time and may also help in spotting leaks should you park in grass or anywhere but pavement where its easily noticeable.
Oh and if it's not already installed, get the sidewall extension option!! They basicly just weld U steel around the top rim so you can extend your space another 2-3'..When/If you end up putting sidewalls up make sure the flat sides are inside and the verticals are outside, a single branch wedged on a 2x6 can keep everything from sliding out.
- Erick