You want a torque wrench that goes at least to 150lbs and is a 1/2 drive. Not 3/8 or 3/4. Most go to 250lbs, but higher is not needed. Most truck wheels get torqued to about 90-110lbs and about the same is true for our 5th wheels. You don't need one over a $100. most are $70-$90. But I wouldn't buy one that cost say a few hundred bucks or the ones that are a few dollars either.
Like Art said, these wrenches need to set back to zero when storing them. Home Depot, Lowes, Autozone and Sears would be good places to purchase one. The click or pop type is the easiest to use.
Also pickup a 4-6 inch extension to use on the 5th wheel and a 8-12inch if you have dual rear wheels on your truck, this is a needed must have as to the lug nuts are set deep in the rim and will also save your knuckles from rubbing on the tires.
Another point is that even though you set the wrench at 120lbs and you go to your lug nuts to start to tighten them and find that some are loose because you will be using the wrench and it will start to tighten the nut, it will turn tighter and the wrench will click when you have reached your setting of 120lbs.
Now, you may come to a lug nut that is already tight and the wrench will immediately click. You will automatically think its set to the 120lbs setting of the wrench. Well, I'll bet you its set much much higher then your 120lbs setting. Unless you've done the torque yourself when the wheel was put on, it very well could be torqued at maybe 180-200lbs or more.
Loosen the lug nut with your lug wrench and now use the torque wrench to set it to 120lbs, and follow the instructions for stepping the torque on the nut, usually 25lbs, then 60lbs and finally 120lbs or what your manual says. Once you get to the 120lbs, that's it your done! Now the next time will be a lot quicker. You'll only have to check for 120lbs. The previous steps are for when you remove the rim or loosen any lug nut.
I also would buy a long 1/2" breaker bar that is at least 16-18 inches long and a socket that fits my 5th wheel and a socket that fits my pick up. If you ever had to break loose lugs with a lug wrench you'll understand why you should invest in getting a breaker bar.
Below is a click type torque wrench.
Below is a breaker bar.
Below are the extensions
