If you can send your retainers to the hightest available 18 hour venture, they are equipped well enough. Sadly I can't tell you if that starts already below the highest one since mine were all ready for the highest ones.

You will notice it, after the 1st quick exploration you do with one of your retainers, you will see a text when he returned that he is good enough to bring back the good stuff. I assume if he is not good enough, there will be a negative text.

Oh and after your first retainer returns and is actually good enough, there is even a new help text popping up.