I can't answer whether it's worth it to you or not. I started with EQ6, but then I bought the Dear Jane EQ software, which is based on EQ5. I found it much clumsier to use. I believe the user interface was one of the major improvements with EQ6, but there were other enhancements as well. If you check the EQ web site they usually list the differences between versions, but I don't know if they go all the way back to EQ5 or not. Personally, I would start out with the latest (EQ7).
You also have to be careful buying older software. It may not be supported, and EQ has had a limitation on how many activations you get on some of the older versions. If the previous owner used up some of those activations, you may have a problem getting it activated on your computer. (If it's new-in-the-box, that won't be a problem.)