When buying an HVAC Rectangular Duct Machine, there are a few things you should consider:

1. Manufacturer: Choose a reliable manufacturer. This ensures that you get a high-quality machine.
2. Machine Type: There are different types of machines available for forming rectangular ducts. For example, the Artizono® Rectangular Machines portfolio includes the Artizon® Ductline, Artizon® Smart-Closer, Foldmaster 20 & 30 Rollforming machine, Lockforming machine, Flanging machine, TDC/F Rollforming machine, Profiling machine, and TPM 2500.
3. Features: Look for features that boost your HVAC workshop productivity and efficiency.
4. Applications: Consider the applications of the machine. For instance, the Spiral Duct Machine is used to manufacture spiral air ducts for a variety of applications, including military industries, trains, and subways.
5. Operation: Check the operation of the machine. The Spiral Duct Machine operates quietly without any significant noise, water leakage, or air leakage.
6. Resistance to Corrosion: The machine should exhibit excellent resistance to corrosion.
7. Cost: Consider the cost of the machine and whether it fits within your budget.

Remember to do thorough research before making a purchase to ensure that the machine meets your specific needs.