The 150MB metric was based on the documentation estimate. I would say that remains correct for my solo instance. The only caveat would be that postegres adds, at current, about 200MB of usage on top of that. Nginx and postfix add just about nothing memory-wise.

This may bloat up over time, or if I had a bunch of users I'm certain it would, but we're not really talking about hosting large communities in this case, so I'd say 512MB of RAM with no other software could probably do the trick as a bare minimum. For those hosting proper communities, 100% future proof and go bigger than that.