I would use silnylon for the floor. Tyvek, not so sure, but its cheap.
A lot of people here like single wall tents.
I prefer double wall, net inner with a fly.
You can get .7 oz netting at thru hiker.
Tru hiker also has silnylon, and .9 oz spinaker material etc.
Quest fabrics has silnyln 2nds, zips, poles, fasteners etc.
Not sure if either ships to Europe, but sil shipping should be cheap slowboat.
About the lightest custom 2 person tent with a fly I have seen was a copy done by Tim Marshall of a coleman tent.
It is really light, like 3#. I will see if I can find it.