Sadly, I missed that topic but anyway.
The usage of open source projects is wayyy more recommened to create a server. That happends because at open source everyone is able to contribute bug report and test. In the other hand, private projects are usually created by one or two developers, and that's it.
All l2j projects are full of bugs, missing a ton of retail stuff, all these are not a one-man job.
Futhermore, you can ask freely for help at (open source)s and actually reciece some answer, but in private projects, at most of the times, the so called developers are busy and by the time they already have money in their pockets.
If you ask me, DONT even think to buy a project. Except the fact that you will propably get scammed, the possibilites will be wayy less that the possibilities that an open source project can offer you.