TDS meters are fairly robust in my experience. Calibration is a good idea, but not a critical issue. If you know your local tap water's typical TDS level, it can be used as one check point. A jug of distilled water can be another check point. So, buying a bottle of TDS calibration solution may not be a requirement. The good thing is that as long as you keep the calibration solution tightly capped, it should have a long shelf life.
Definitely check the TDS level of RO water you purchase from a vending machine. Any breakdown of the membrane will mean that you will be left with water that is not as ion free as you expect. A TDS meter is an easy way to check the machine's output.