If unfinished, find a downstairs roughed-in plumbing pipe for a sink, toilet, floor drain, etc. and then wire an outlet near that. Buy tubing, adapters, etc. and connect to a good capacity dehumidifier and keep it running. One with an adjustable humidistat is nice.
I did that in my basement and it is always below 50% in the summer and 30% or so in the winter....it will work for mold, and rust (I keep my guns there).
If any of your guns have wood ‘furniture’, be careful not too keep the humidity TOO low. I’ve heard it can cause the wood to dry out and crack over a long period of time if the air is too dry. I don’t have this problem as ALL of my firearms have black plastic/polymer that makes them look “dangerous” to Liberals and idiots(please pardon the redundancy).