What should one know before buying a new HVAC?

• Looking to replace our split system. Condo is ~1300 Sq ft.

• What are some unnecessary markups (or gimmicks, if they exist in this industry) that one shouldn't fall for? Eg UV light filters, etc.

• Are there qualities of any particular system, or upgraded component to a system that are worth their additional costs?

Any advice or insight is appreciated!