Rule #2 – Wants vs. Needs:
Is what you are considering a WANT or a NEED? WANTS are limited by desire and money. If you want it and can afford it you should have it. NEEDS are driven by necessity and are more difficult to address especially if a limited budget is involved. In some cases, a blend between your wants and needs can be the most reliable and cost-effective avenue to pursue. For example: If you are in need of a different reliable Nav/Com with Glideslope AND you want (or need) an approach certified GPS, for a fraction of additional cost, you may consider a new GPS/Nav/Com rather than a good used Nav/Com and a separate used approach GPS. The difference in cost will provide an all-new product with a manufacturer warranty and can provide more user functionality.

Rule #3 – Buy by application:
So much of today’s options are driven by the type of aircraft, the capability of the pilot, and how the pilot intends on using the aircraft. This issue becomes most obvious when discussing GPS, Glass Panels, and Autopilots. I don’t think you will find much argument if it were suggested that some form of WAAS GPS is almost a necessity these days. A VFR “weekend flyer” in a small single can obtain a high level of situational awareness (especially with the restricted airspaces) and value from a handheld GPS. A practicing IFR pilot who intends on tackling IMC conditions whenever possible (particularly in business applications) will rely heavily on a WAAS GPS. The choice of product, even in this realm, can vary between a GPS with a high level of intuitiveness but less overall capability and a GPS that requires a much higher level “learning-curve” but contains more potential operational applications. The point is this: You owe it to yourself to do some research and seek the advice of a knowledgeable “expert” to determine what will best suit your application now as well as any considerations for future applications.

No matter what you are buying, avionics purchases are not inexpensive. In most cases, you will probably require some installation work associated with your purchase. Your safest bet is to purchase your equipment from a respectable source because they are most apt to stand by what they sell. Keep in mind that many shops will not install equipment they didn’t sell and/or may charge a higher labor rate to install such equipment.