Gotta use card stock for the reason that it will hold up to its shape. And if you plan to glass it and make it hard again cardstock is needed, paper wont hold, think of it as putting water on paper its going to just loose its rigidness.
as said you need card... ive done alot of pep work... and found 200gsm card was ideal as its easily folded/shaped but still strong, and probably the miost acceptable thickness for the printer...
remember you need to take your printer into account.... i suppose you could get away wtih 160gsm or 180, but as i say i like working with 200.
using payer is simply a no no... i will not retain any shape, using cardstock the end product will usually support itself... we just ad inserts to hel keep the proper overal shape for when we resin/fibreglass!!
if you fibreglassed onto paper i can see it all just collapsing in to a heap lol!