You need to determine the amount of support offered, and the cost of that support. You also will need to check on the availability of the support. Support is not worth very much if it is not available when you need it. Check the office hours of the support staff. Knowledgeable support is also important.
Some companies support is worse than no support.
Ask the company for names and phone numbers of people who use their program if support is very important to you. If at all possible try the program before you buy it. You will be using the program for a long time. You need to be sure that it is indeed what you want. Some companies have a working demo version that they sell or give away.