Those are the problems you will run into indeed.
If you wan't them in a separate building, insulation, waterproofing on the inside and heating the room are the way to go.
x2 but try to leave a gap between shed & insulation to allow an air flow.
Space heating is by far the best to prevent condensation problems, you can always heat certain tanks as required with aquarium heaters.
Heat the room to the correct temperature so the tanks don't need heaters. This will reduce evaporation/condensation.Space heating ?
I think we scared him enough
Beware:
Polystyrene & PVC cable coating react together, it takes a good while, years but worth remembering.
Mice just love to burrow & nest in polystyrene.
I know this from experience when I dismantled my old shed years ago.
Check @frederick thompson 's Journal "Freds new pond build". I think around page 24/25 he gets a shed and (around page 30) starts modifying it to hold a big quarantine tank.
Oh wow that pond / shed setup is insane