Agree with barry, i would pull the seats and carpet and assess the extent of the rust. I would say from the pictures you've posted that you should probably have that welded. It is a structural part of the car, and since the seatbelt buckle is bolted to the seat, you especially want to make sure those parts are structurally sound.
You can buy rust-repair panels at several places, such as
millsupply.com, and they sell floorpans. As far as cost, I would try to find some local shops (autobody, welding shops) that would be willing to weld the pans in for you. If you find a place that is willing to let you prep the car (by pulling the seats, carpeting, wires, etc. from the rust areas) and supply the panels, and just pay them to cut out the rust and weld in the new metal, that would help you save some money.
This would be a good time to also check the car for rust in other common areas, such as behind the front fenders, other parts of the floor, etc.