When you are looking for a flight instrument there are certain obvious considerations like features, size, weight, reliability, warranty, battery life, etc. that will effect your decision, however there are other very important factors you should also consider.
If you are buying a new (or used) instrument make sure you are getting it from an authorized dealer. Products that do not come from authorized dealers are gray or black market. Usually they do not include instructions in your language and they will lack local warranty and local customer support. Buying these products also supports illegal and unethical business practices and is very damaging to our sport and in the long run, causes the price of these products to go up. Whenever possible buy from your local authorized flight school or instructor. They provide valuable service and support to their customers and the health of our airsports depends greatly on them. If you are uncertain if your local dealer is authorized call the product distributor or manufacturer.
There is a lot of good technology out there but what do you do when you need help learning how to use or understanding it? What do you do when you need a repair? Will you be able to call or email someone experienced and knowledgeable to get the help and answers that you need in a timely fashion or will the answer that you are looking for involve several unanswered emails, followed by several unanswered voicemails. Once you connect with someone will you get friendly knowledgeable answers or will you get a bunch of “maybe”, “I don’t know” and “I’ll get back to you”. Before you invest in an instrument find out about the reputation of the local distributor.
What good is a flight instrument when it is broken? Find out if the product you are considering has local service or if it will have to be sent to some far off place for "you'll-get-it-when-you-get-it" service. Products that must be sent out of the country can take 6-10 weeks (or more) to come back from repair. Typically there is considerable shipping cost and increased risk of loss associated with overseas repair. This is an easy one to check - ask your local professional HG/PG school, dealer, flightpark or instructor about the service time on the product you are considering. Also check with your fellow pilots and find out what their experience has been with the vario that they use.
Many instrument manufactures, importers or distributors have changed hands, discontinued the product or worse gone out of business (some more than once). If any of these situations occur while you need service or support you will find that it will be very difficult to get the service that you need, or worse, it simply will not be available. Before you buy a vario do a quick check to see how long the manufacturer has been in business, how long has the distributor/importer been in business and check to see if they have "orphaned" instruments and customers in the past. The continuity of the manufacturer and the distributor is very important to the resale value of the vario. The less stable the manufacture/distributor the worse the resale/trade-in value.