1. If you are intent on heating the garage, you'll need to insulate. That's walls, floor and ceiling. The cheapest to run electric heating is an air sourced heat pump (or air con). Expensive to install. Www.cooleasy.Co.uk do good quality dehumidifiers... did a thread a while back. Putting one under breathable cover seems a reasonable comprise.
Sorry to got off thread @AirCon but how do you rate air source heat pumps as I’ve just finished building three 7 bed houses for clients in a small devel and they are all saying the air source is shit and the houses are cold. I also put logburners in which they all Use. This was apparently the future