I just purchased a trailer for my RC needs. Now I bought mine used so I got what was there. However, I did a HUGE amount of research before I purchased mine so I knew exactly what I was looking for before I purchased it, so I was being pretty picky in the used trailer I got. My trailer is a 6'x12' made by Continental Cargo.Here are a few of the features that I was looking for that I think will be very pertinent to what you are asking on here.
[*]A side access door in the front of the trailer. This was a deal breaker for me and should be for you as well. Among the many reasons for having it are it makes is easier to secure the planes down as you have access to both ends of the planes as you are storing them. This will also help make your trailer more usable as you have more usable storage area now. I use the front portion of my trailer for storage of chargers, radios, tools, fuel cans, a small generator, my folding chairs, my pop-up tent, and any other thing I might need in my trips to the field. I also have built in a small fold down work space so that I have a place to do repairs at the field. If you're going ot have a trailer you might as well make sure it's useful in more ways than just hauling your planes to the field.[*]Trailer height. While this one may not seem important think about it before you dismiss it. My trailer was built with an extra 6" of height in it. Because of this I can EASILY stand up in my trailer. Trust me that this will make life so much better for you, especially if you have the front door in the trailer. You don't want to have to always stoop down when you step into the trailer. I can stand up easily inside of my trailer and it makes using it so much easier.[*]Back ramp. This is extra on many trailers, but for RC uses I think this is very important. having a ramp will make getting larger planes into the trailer easier.[*]Electrical. seriously consider adding a 110v/12v electrical system to the trailer. I have a 12v system in my trailer that powers an inverter to provide 110v inside the trailer. I also have a converter/charger that are used in RV's. This takes whatever voltage coming into it and powers the 12v system of the trailer as well as recharges the batteries. So when I am at home I plug into my house electric and it powers the trailer and recharges the battery at the same time. I haven't done this yet, but will be adding a solar panel on top to help save electricity and keep the battery charged. I also have a small 800 watt generator that I picked up at Harbor Freight for $90 for 110v power when I am at the field.[*]As for the spacing of the wall studs. I really don't think this is going to make much of a difference.[*]Plywood lined walls. Absolutely a must. You are going to want to build some sort of structure inside to haul your planes in, and the plywood on the walls will make building anything that much easier. Yes, you could add later on. But why not have it done for you when you buy it?[*]Screwless exterior. In my opinion this is something they can charge you extra for that really won't make much of a difference at all[*]V-Nose. If you can get a V-nose trailer don't pass it up. This will give you a little bit extra space for storage inside.[*]Hitch box. You'll probably have to add this on yourself, but adding a hitch box will give you some extra storage space. I have my 12v electrical system mounted inside this box on my trailer[*]On the security side of things. Of course it's impossible to make anything foolproof, if somebody wants your stuff hard enough they will find a way to steal it. So here are a few things you can do to help keep it all secure. First, do NOT (absolutely not at all, please don't even think about it) put anything on the outside of your trailer indicating you have RC equipment inside of the trailer. Even putting a graphic of a plane, or your club logo on it can identify your trailer as storing RC. Doing this make the trailer a target. Thieves look for these kind of trailers and will snatch them if they get half a chance. Heck, I'm considering putting "Acme Septic System Maintenance" or "Tony's Diaper Service" on the side of my trailer so that the thieves think twice about breaking into mine 
. When my trailer is parked at my house I have a ball lock on it, A lock that goes up into the ball portion of the hitch and prevents it being hooked up to a vehicles, as well as a lock on the handle of the ball hitch. Also make sure that if you have a receiver hitch on your tow vehicle that the pin holding your hitch in place is of the locking variety. It doesn't do any good to have the hitch locked down if they can simply pull the pin and slide the whole hitch out of the receiver on your truck, and then it's bye-bye for your trailer. For locking the doors on the trailer I make sure I use high security type lock, which are harder to cut. And lastly, if you can when you have the trailer at home chain it to something so that it's secure and can't be rolled away.[/list]Ok, I think that's about all I can think of right now. Because of many of the things that I mentioned above my trailer has actually turned into an extra room for my RC stuff. I keep my planes stored in it at the house and it's freed up some room in my garage. Sorry that I don't have any pictures of my finished trailer right now as I'm not quite done building it out like I want it. So I just haven't taken any pictures yet. I'll get some shot when I finish it out. But here are a few pictures of the trailer like it was when I purchased it.
Ken