40ft dual tandem axle (12k) are the most useful hotshot trailers in my opinion. Straight deck no dove tail.
I would disagree with it being "the most useful hotshot trailer". However if your doing hotshot work for the oilfield and just the oilfield, that's the best trailer in my opinion. I don't like those trailers for anything but that type of heavy hauling, equipment, pipe, drills, ect.... because they are so damn heavy. A 40' tandem dual gooseneck by itself weights around 8k - 9k. That is the main reason for other hotshot work I would recommend a triple axle "bed between the wheels" gooseneck with a 102" wide deck, lower and lighter overall trailer.
As far as the truck goes, I would bump up to a dually. The trailer you need for this kind of work will push a SRW all over the road, not to mention I wouldn't want to lose a tire hooked to the trailer. Sure you can do it with a SRW, but breakdowns or accidents cause down time, unexpected expenses and can ruin your clientele.
Also you can legally haul up to, but not over, the weight rating of your truck and trailer axles. So as long as your under the weight ratings of all of your axles and under your tire weight ratings then you are legal.
I'll second what Spatel said above me though, you have a lot to do before you get started in order to do it legal.
-Liability insurance
-Vehicle/ Cargo insurance
-DOT numbers
-MC numbers
-Annual DOT truck and trailer inspection/ inspection sticker
-I believe an IFTA sticker for texas and any other states you travel, if your over 26k.
Take my input with a grain of salt, this is just what I've learned and how I've interpreted the rules. Others might say something different. Just do as much reading as you can to properly prepare yourself so you don't go in completely blind.
Here is a sight with some good info, not all the info is good and I don't really agree with everything posted over there but there aren't to many places to read about this type of info.
http://ltlhotshot.myfreeforum.org/index.php