The card stock I use is 200-210gsm weight, that’s usually the highest weight a home printer can take without it being too much for the printer to cope with.
The resin and bondo/body filler I recommend you look online and buy in bulk off eBay, if your building a whole suit especially. You’ll save a lot, even if it does seem pricey at times. I used fibreglass mat too but that’s just because I found it really cheap, otherwise I would use fibreglass cloth because it’s woven and really makes it far stronger. If you live I the uk then take a look at halfords because they have basic supply’s for all of these materials, if your in the U.S. then I recommend Home Depot.
Hope this was of some help.