There are several things to consider before buying a CNC surface grinding machine:

1. Workpiece: Consider the size, shape, and material of the workpiece that will be processed by the machine.
2. Surface finish: Consider the required surface finish for the workpiece and ensure that the machine is capable of achieving it.
3. Accuracy: Consider the required level of accuracy for the workpiece and ensure that the machine is capable of achieving it.
4. Production volume: Consider the production volume and the machine’s ability to meet it.
5. Automation: Consider the level of automation needed for the machine, such as the ability to load and unload parts automatically.
6. Space: Consider the space available for the machine and ensure that it will fit in the designated area.
7. Maintenance: Consider the maintenance requirements for the machine and ensure that it is easy to maintain.
8. Cost: Consider the cost of the machine, including the initial purchase price, operating costs, and maintenance costs.
9. Support: Consider the availability of support, spare parts and service from the manufacturer.
10. Brand: Consider the brand reputation of the manufacturer and their history of providing quality machines.

It is important to do your research and carefully evaluate your specific needs before making a decision on purchasing a CNC surface grinding machine.