The wheel or wheels of your mobile robot are very important. A wheel usually consists of rubber, so as to have big rubbing force with the ground and spin without gliding at the ground. There are two qualities of rubber on the market. Usually, the cheap wheels are made of rubber that is quite hard to bend and glides a bit. The other rubber material is more real rubber and it is quite good for that kind or robots. It does not glide at the ground even if the material is glass or tiles. Unfortunately, there is no way to understand how good the rubber is using your eye without touching the wheel. So, keep that in mind when you buy your wheels. Both wheels described earlier have a B-shape hole and it is used to connect the wheel with the motor. You can see this in the following image:

The cheap option here is to use plastic gear motors, plastic wheels, and plastic connections. It is easy to understand that plastic things are cheaper than iron ones. So, yes they will do the work but, if you have a good budget, go for iron. An example is shown in the following image:

Obviously, you will not see a huge difference in the mobile robot's total behavior. However, it will be more stable, without so many errors or or much need to balance your motor wheels and other stuff like that, which is actually extra pain in the overall project.