I've ordered a RPi 3B+ with 500 MB RAM, as that was all that I found that evening.
My google-foo doesn't find the VenusOS minimum requirement, and the RPi space here doesn't have so simple a question. In the event that 500 MB proves inadequate, it's worth knowing what's required. The microSD card coming with the SBC is 16 GB, so that'll do. But how much is loaded to RAM at runtime, is the question.