If you really want to save money, go and have a look around the local works units. There are often companies that get regular deliveries of items and the packaging is a liability. If you can find one with big sheets of polystyrene, they may be happy to put it to one side, intact for you to collect. And free.
I did this with a local kitchen unit supplier and that lined most of my old prefab garage. Lined with hardboard that just glues to the insulation, it;s still good, 10years on.
For summer, I can't recommend a safari roof enough. (That's what I call it, as that was what land rovers had.) A secondary roof skin with an air gap.
My garage had a simple ply and felt roof, which was murder in the summer. When I added a second roof, as the original leaked, it made the place really pleasant to be in, during even the hottest summer days.