Interesting thread, and I would agree that desiccant dehumidifiers are the way to go if you suffer from high humidity.
In my 4x5m workshop the wall/roof construction is outside -> in
cedar shingles ->breathable membrane -> 18mm ply -> 80mm studs infilled with foil backed Celotex ->15mm air-gap -> vapour barrier, 6mm ply.
The floor is suspended, 'exceeding building standards' and insulated between the joists sitting on 16 concrete piers with 18mm mdf plus ~25mm pine T&G floor boards to finish.
The inside is pretty much airtight - when I vented my chip extractor ~380l/sec, through an external vent I have to remember to set a window ajar to let air in - even trying to open one of the double doors whilst the extractor was on was difficult because of the vacuum generated inside.
The principle reason for that construction was to control noise - having suffered from abusive neighbours at my previous property, and needless to say in the 12y it's been in use I've not had one complaint!
The other upside is that I have absolutely no issues with anything rusting, be it cast iron table tops, hand tools or even a 1x2m metalworking table with a tack welded 1/4inch steel top - all completely bare steel and no rust.
So if you pay attention to both moisture ingress and egress into the building structure then other preventative measures aren't IMHO necessary.
I also have some pretty heavy machines - 600kg metal lathe and I've no idea how heavy but more than the lathe, cast iron spindle moulder and have no problems with the suspended floor sagging albeit they are pigs to move around on my own and there is the odd creak whilst doing so!