Sixstringsteve,
At 18,000 lbs you are probably going to need a CDL. 26,000 lbs is the limit under your standard class D drivers license. To keep things all legal you are also going to want to check into insurance for such a load and your upgraded licensing as well. Any modern heavy duty turbo Diesel will haul the weight but, a medium duty truck will do it more efficiently than your standard 1 tons. This means Furd F-450+, Dodge Ram 4500+, and the GM Kodiak and Top Kick 4500+. Dually is the only way to go and 19.5" wheels are going to be the norm for these trucks. For the Ford in your price range it will most likely be a 6.0 powerstroke. Ram will be either the 5.9 or the 6.7 Cummins. GM's will of course be the 6.6 Duramax. The Duramax suffers for overheated fuel once the fuel level gets around 1/4 tank or so, and the instrument cluster likes to fail and the motor likes to eat injectors. Ford 6.0 from the factory is not a great choice but, it can be made into a very reliable motor if you are willing to sink some money into it. The Cummins 5.9l will be the best engine but, the rest of the truck leave a little to be desired. Interiors are not as good as the other two offerings. The 6.7l Cummins is also a great engine but, you are going to have to be willing to spend money on it just like the 6.0 Ford albeit not quite as much.
All of the big three make good trucks but, they all have their weaknesses also.
You may want to look into a RV type semi truck. One that doesn't have the second rear axle. Truth be told those rigs are only built to haul often, heavy, and reliably. Volvo and Freightliner are two that come to mind. Using a pickup truck while capable will only wear out more quickly but, you do end up with a truck that is more multipurpose.
For towing experience...... You just have to do it and learn from your mistakes. Riding with someone while towing will get you some knowledge but, the best way to learn is just to do it. Perhaps start with a smaller trailer so you can get some of the basics out of the way. It would suck to be learning with an expensive 5th wheel RV behind you.
LT.