As far as trailer hitches that fit. I believe the Bushtec gen II hitch will fit. I pull a bushtec trailer with mine. Dauntless uised to make hitches that fit the gen 2 also. But I don't think they do anymore.
Depending on your tongue weight you will wnat to upgrade the suspension probably sooner than later. I know that GP Suspension is interested in working with a gen III owner to develop a complete cartdiged kit for both front forks. You may want to call them and see if you are looking into suspension. They have done systems for other brands with similar front fork arrangement.
I would recommend a Power commander V with auto tune module. It will give you power when you want it, but also gives you good MPG. It gives you the ability to have two maps loaded so you can have a power map and a economy map with the flick of a switch. I usually average 42 MPG two up towing the trailer and I am not to gentle on the throttle. I tend to play in the twisties as much as possible. The PCV works very well when towing. The stock brakes are very good and no reason to upgrade there. Even towing and riding agressively in the twisties, they provide plenty of stopping power. As you know when towing the transition from brakes to throttle is where the trailer pushes you and you have to account for that when making your turns.
I have the Remus exhaust cans on, but it is purely for noise candy. They sound great, they may add some power, but I have not done any dyno runs to verify.
The are some frankentank threads on here. I have the second one that dirtracker30 did. Basically he took a stock tank and modified, so it looks stock, Mine holds 9.6 gallons. He shows some very good pictures of how he did it, so if your inclined it is a great way to add capacity and keep the gas up front and easy to access. The tank extends my range to about 300 miles with plenty in reserve to find a gas station.
EDIT: here is the thread by Dirttracker 30 building my tank
https://www.fjrforum.com/forum/index.php?showtopic=135505
here is another more recent thread showing more of his process:
https://www.fjrforum.com/forum/index.php?showtopic=149373
here is a picture of the tank when I was painting it;
A side view mounted on the bike:
Another shot with the trailer hooked up
A different angle
From your GW pictures, I can't wait to see what you do with the electrical for all the farkles. I would like to know what switches, indicator lights and relays you tend to use. They look really clean.