Absolutely 1,000% no way to go but the dual axle. I towed mine on a 5 x 10 3,500# single axle and while it is under the weight rating, the single sidewalls flex too much for a comfortable tow. I upgraded. If you get a single axle, I PROMISE you will not be happy with it's performance.
I know you're stretching your tow limits a little, but I would strongly recommend a 16 foot. I have a 20 foot dovetail, which has effectively 18 feet of deck. Tractor, FEL, BH, and box blade and I've pretty much filled it up. Sure, I could let the BH hang onto the dovetail and such, but I'm quite certain that you'll feel the 14 foot length a little tight. You'll be fine with the 16' though.
The biggest thing here is getting the tractor positioned just right so you have the correct load on the tongue. Having those extra 2 feet will make a big difference in safety, IMHO.
I towed my 24' Bayliner with my 4Runner. Surge brakes. The boat and trailer together was a solid 30 feet long and pushing 6,000# with fuel, water, and adult beverages on-board. I couldn't go over 56 MPH or get over 4 MPG, but it hopped along for the 5 miles I needed to tow the boat.
I think you'll feel the rig behind you, but you're VERY smart going with the brakes. They are CRITICAL to safety. Personally, I'd get the WD hitch. And after towing my 20' for a couple months now, I'm getting sway control as well.
I know it's a bite, but I promise...I've been down this road. Get the 7,000# trailer with brakes. Your truck will be OK.