Have you checked the price of polycarb lately? Your going to need good stuff on the roof, to support snow when it comes. Its eye wateringly expensive. Triwall is reasonably insulating, but not as good as a couple of inches of kingspan or fibreglass.
I'd say put a solid roof on, out of corrogated, or whatever, and insulate it well.
I painted the inside of my shed with very bright white kitchen and bathroom paint. The difference it makes to the light is unbelievable, its like having another striplight in there.
By insulating the roof, it will make it a lot nicer to work in when it is pouring with rain. You will need earplugs under polycarb, coz of the racket it makes.
If you rgoing to put a concrete floor in, yu may as well put two inches of kingspan under the concrete. it will make a huge difference.