The less expensive Park stand is a single sided stand. You need to take the wheel off and remount it the other way around to check for dishing.

The cheaper Spin Doctor stand checks for dishing (rim centering) at all times. (it is worth the extra $10 to buy the calibration tool that is used to check the center of the guage.)

The plastic parts are the forks that hold the wheel. Do not crank down on the skewer any more than just snug and it will last indefinately. Unless you are going to do a lot of wheels, save the money for wheel parts and get started.