Is phpbb or Mybb still good, or should I look for paid ones?

[batman]

The thing is phpbb and mybb both needs constant attention. Otherwise, you will be bombarded by spammers.

Xenforo pretty much runs itself if optimised correctly.

You are right, phpbb has a great community. But all those features of smiley and other what not's will not be available in it.

If you are looking for a free solution solely then mybb is the best option for you. Specially the features that you asked for. The url feature - "I am guessing that people will insert url & it will be converted into thumbnails with preview?"

All those features more or less are present in mybb. But you will need paid theme to execute correctly everything as per your needs. And also it is highly recommended that you use a VPS for your forum. Because most shared hosting companies don't bother to give all the features that you are going to need. You will need patience and to some point assistance in code (if you don't know coding) to get things working.

And if you have money then go for Xenforo. It's the best. It will come with all the features right out of the box.