IMO the 15" tire would be a better way to go, having a higher load rating and less rotations per mile leads to better reliability in the long run, and I think they look better.
Our Xterra came from the factory with the tow pakage, so all I had to do was purchase a brake controler and dig out the wiring harness from under the dash, wire it up, and mount the controller.
Dealers. You need to decide what is most important to you concerning the dealer. Is it price, trade in, service, integrity? One truth about dealers is they give preferred service to people who buy from them. You might find a better deal at a dealer in another town or state, but if you ever do require service, how much trouble is it to get the unit back to them for service? Are you willing to pay a bit more for the trailer from a local dealer for the added benefit of ease of service, if you ever do need service? We bought our trailer from Ruthruaff RV, a small, family owned dealer who specializes in Ultra Lite Trailers. They are in Tucson, we live in Phoenix, 100 miles away. To get the trailer to them for any issue, is at least a half day commitment from us, and they only accept service dring the work week. I really like the people we purchased from, but it is an added hassle. In the end, for us, it was worth finding a trustworthy dealer. Your mileage may vary..
Good luck man, your trailer is out there waiting for ya!!
Hope that helps, any other questions, let me know.