The best advice I can give you concerning a tow vehicle is do not skimp. Many people try to get by with the smallest (cheapest) tow vehicle they can. I think this is a mistake.
I've heard people say that their tow vehicle can pull their rig just fine. On one of the Toyota commercials, a sales agent mentions that the Tundra pulled the space shuttle. Yes, it did. But that is somewhat misleading. Lots of small tow vehicles can pull weights they weren't designed to pull. That doesn't mean you should do that. There are safety issues involved here. First and foremost, remember that your tow vehicle is also responsible for stopping what it is pulling. And what happens when you come to a steep grade, or even a slight hill? Will you be able to safely pull the rig up such an incline? Will traffic back up behind you, impatient to pass and doing so in dangerous places? And if you constantly tow a rig that is too much for your vehicle, what damage will it do to your tow vehicle?
Be leery of advice from RV and vehicle salespeople. They are, after all, simply trying to sell you something. Instead, get online and look for reliable sources. Most RV brands have forums where you can get some really good advice. For example, I have a Rockwood trailer, which is made by Forest River. You can find specific forums for their numerous brands at http://www.forestriverforums.com/forums/. Before you purchase a trailer, spend time perusing the forums for advice related to towing. If you join a forum, you can then perform searches for specific information.
I find towing to be the least enjoyable part of the RV lifestyle. Bad weather, high winds, hilly terrain, narrow roads, busy highways -- all of these are difficult enough when you are towing with a good tow vehicle; with an underpowered one, these can become nightmares, and deadly.