I edited my above post while you were replying (I'm bad for that), so please re-read it.
You'll have to get custom if you want Reef Ready. Otherwise you'll have to do the overflows yourself, or buy a used one with them installed already. If you have patience you should be able to find one in the next 6 weeks or so. They're always popping up. When I was pricing out cubes I
think Seastar made a 30x30x18" for $300ish...? But I'm pretty sure that is a "custom" size.