hey, youre half of the way there, you have a drilled tank and base rock.
I'd get a sump and a skimmer and then start.
if youre handy you could probably rig something up with what you have if money is too tight to go new. You can also make a sump from another fish tank. How much room do you have underneath? You might be able to fit a 20L or a 29g and then partition it off to whatever compartments you want.
I'd go intake with skimmer section, middle make the refugium and then a bubble baffle and the return pump section.