"And lastly, do I really need some super expensive scissors? Wouldn't it be possible to use some cheap gardening scissor?"
I tried to use generic scissors on some course 100% kevlar used in canoe repairs. I got nowhere! Not 5mm. A steel wheel cutter did nothing, just folded the fabric. The $35 scissors, supposedly for kevlar, were done after about 2 meters.
So, YMMV, but this is one place where a specific tool is worth the $$. Get the fabric, go to the shop, and see if what they have will actually do the job. My local shopkeeper was shocked at how ineffective her "excellent" scissors were.
Or, have your supplier cut the pieces you need.