Definitely get a 3/8 and possibly a 1/2. Torque wrenches are best (most accurate) near their center range. Park's good, but a little pricey. Most auto parts stores would carry decent ones for half the price of the Parks. I just use old craftsman ones.

3/8 will take care of 90 percent, 1/2 for BB's and Lockrings (although 3/8 work fine on those too)