Fenders from scratch can weigh just about anything you want them to, I would try and make them as light as possible. I typically made them from Balsa for the lightest possible fender. Nowadays I use John's GEE fenders.
If room permits I would use 4 fenders, assuming this is a 4 wheeled car. I glued my fenders on, no pins. Make it as simple as possible to install the fenders.