I'd start by pulling the carpet and go from there. The factory undercoat can hide damage and you usually get a better idea of the damage from the inside (under the carpet) than looking up from the outside.
A whole floor pan means the least welding but is very big and awkward to handle and position in the vehicle. Pan sections (driver front, pass front, driver rear, pass rear, cargo, and trans tunnel if necessary) are easier to handle and require less welding, and are often a better option if the whole floor isn't rotten (especially if the trans tunnel is in decent shape). Individual plates to patch every hole are the cheapest option but will require more welding and some bending to fit right.
It also looks like you have a rust hole in your frame as well, plate is usually the best way to fix that.
Welding the floor isn't too difficult if you have access to good equipment that can weld thin metal (the floors are 20ga/22ga). It can be done with a cheap welder but is much more difficult since most cheap welders don't have very precise adjustments which make it difficult to weld thin metal without burning through. Most cheap welders (HF, etc.) aren't designed for anything thinner than 16ga/18ga.