My shed/workshop is wood and it's clear that moisture penetrates it from the amount the de-humidifier collects when it's damp outside. With the de-humidifier and heater (thermostat set very very low) I've never had problems with corrosion, unless you count the time a bottle of ferric chloride dripped on the lathe!

Soundproofing is an issue when the router's running, but the other machines are quiet enough. For that reason I try not to run the router after 9pm, though it's been known for me to still be up there well past midnight...

I wasn't aware of this within 2m of the boundary regulation. Mine is less than that as it's only about 0.4m from the hedge. I think the 20*10' structure cost about £2000 with delivery and installation, plus the cost of the concrete base and insulating it. Maybe it's worth asking some suppliers what they can offer?