by "glass panel heaters" do you mean electric heaters?
As kitchenguy says, all electric heaters are equally efficient. They all turn 1kWh of electricity into 1kWh of heat. You may as well buy the cheapest safe product.
My preference for electric heaters is oil-filled radiators fixed to the wall. They are safest because they don't get hot enough to start a fire or burn you, and they give a more regular, comfortable heat.
Also consider Tube heaters fixed to the wall at skirting level. They are low power and will prevent damp and frost.
You need to have two thermostats to control the heating. When unoccupied, about 10C will keep it dry and prevent frost. You don't need a timer for protective heating, just a thermostat. On mild days it will not even turn on, but it will start up on a cold night. If you are in England, it's very cheap to keep an insulated building at 10C.
If you are going to be working in there every day, you might want a timer for the comfort heating, to come on (say) half an hour before you'll be going in there, and bring it up to something like 20C. Running comfort heating when not needed will be much more expensive.
If you have off-peak electricity available in the house, storage heaters will be cheaper to run, and floor heating becomes more realistic, because the block of concrete will retain heat. You can often get storage heaters for nothing on Freegle from people who are upgrading. They are very heavy because the contain bricks. They are made so you can take the bricks out to carry and transport them, you take the bricks out, and put them back inside the case when mounted in their new position. You will need a qualified electrician.