If you go with the volitans, I would only add one. They require 50 to themselves for one adult fish. Lions grow pretty quickly also.
You might be able to keep one other fish in with the volitans. As long as it's going to stay large enough not to be a snack. Maybe one of the larger angels or a foxface (also venomous).
Landscaping should have caves and resting spots. Lions like to hang upside down under the rockwork.
A sump would help, can't help you with a design but if you do a search for DIY
sumps, I'm sure you'll get lots of info.
Just as a side note, (and some may disagree) with a 30-50 gal
sump, you might get away with keeping two volitans in the tank.