Lots of good advice. The only thing I'll disagree with Andy about when it comes to putting some money into your Dakota is brakes. Having lost brakes on a trailer before, I can tell you from experience that you want to make sure that your tow vehicle can handle whatever you have back there. If you start putting money into the existing truck, I'd seriously consider budgeting some of it for larger rotors etc....
V-10/Hemi engines: Take a good look at what you are getting into, other than a lower purchase price. They are fuel hogs, or so I've been told be people I consider reliable. Purchase price for a diesel will be higher, fuel cost is higher, low end torque is higher (good thing), engine life with proper maintenance is higher (good thing), fuel economy is higher (good thing). I'm a Dodge fan, and love the Cummins. I've had fewer problems with mine than friends have had with their Powerstrokes. Fuel prices have mine parked right now, but that just means that I'll have it longer, and have it when I need it longer.
If I were in your shoes, I'd sit down and figure out exactly what I planned to be pulling and be honest with myself. I'm talking about dimensions and weight (when finished out and loaded). Add about 10% to the weight when you are done. THEN I'd start looking at my needs for a tow vehicle.
The last bit of advice is from a guy that bought a 1 ton gasoline driven truck, before he bought the gooseneck horse trailer years ago when he was much younger and more impulsive. I pulled that trailer once, and traded for a diesel.