Do I have to buy one of the featured base products or does logos allow you to customize a package (e.g. only interested in having KJV, RSV, ECF, a specific dictionary, one commentary, etc)?
The software is free. Many resources can be purchased individually and will work. But you get additional functionality in some of the higher end base packages - plus many more resources for your money. You'd need to look at the cost of the resources you would actually use and make your decision based on their cost.
