Before you even decide on buying a unit, first you want to do your homework on both the equipment you are purchasing as well as the company that is going to do the install for you. Once you have picked out a qualified HVAC company to install your new air conditioning or heating system there are several things to be aware to ensure that your install goes well and is a quality install.
Your contractor should make sure that there is enough room around each component to allow for follow up maintenance, service, or even simple checkups. If you do not plan on using the install company for maintenance services, it is extremely important that whoever is going to be following after the install is able to access the equipment in a normal fashion.
Although some may take it for granted, you should have the techs who do the install show you how to clean and change the filters, because while most units are fairly straightforward some of the higher end units may have slightly complicated mechanisms that need to be properly explained.
Any outdoor component such as the condenser absolutely needs to be protected from the elements and vegetation as will specify by the manufacturer of the unit. The quality HVAC professionals are well aware of this necessity and will make sure that your outdoor components are properly sheltered, covered, or able to tolerate the elements where your home is.
One more install tidbit that some people tend to overlook is to make sure that the indoor and outdoor components are using compatible parts and proper coolant charge levels when only one aspect of central air system is being replaced or fixed.