Okay, I'm not the most experienced, but I had the same questions starting out!
1. Interfacing is a stiffener - it helps your bag hold it's shape so it doesn't just fall in a heap when you put it down.
I can usually find it pretty cheap where I live. There are different kinds ofcourse. I used to use the kind I had to sew in, but then tried the iron on kind and it's fantastic! You just put the shiny side of the interfacing against the wrong side of the material and iron on and it sticks! Nope, it's not just for bags - some clothing calls for it also - collars, waistbands, etc.
2. I have made a tote with a sturdier bottom by cutting a piece of cardboard the right size and wrapping it in clear packing tape. Then I just slide it in between the lining and the outside when I sewed them together. I didn't really care for it much this way myself. My totes seem pretty sturdy by themselves, they just don't have that flat bottom look when they are empty.
For straps, I've used just material folded and sewn with or w/out interfacing and I've also purchased the nylon webbing stuff and used that. Either way they haven't broken on me yet.
I hope this helped a little bit!