Sounds like you're headed in the right direction. Here's a couple of random thoughts.....A buddy of mine who welds a lot buys Hollywood bed frames at garage sales and thrift stores, for 5 or 10 bucks when he finds them. Great cheap source of angle iron. Frame up your shelves, make racks to slide them on, make doorways for the smoker, etc.
Foe others who read this thread later, shelves from old ovens are OK. Shelves from old refrigerators are not recommended, as a lot of them were cadmium plated, and can be dangerous when heated, or when the plating starts to peel.
To finish your expanded metal shelves, get your barbecue grill good and hot, put them in for about 45 minutes before you go to bed. Turn the grill off, and let them cool overnight, This will remove the oil and rust that may be on them. Spray them with some Pam or other unflavored cooking spray to prevent new rust. When you season the smoker for the first time, spray them again, and leave them in to smoke up. They should stay rust free, and be food safe.