Use a USB HDD I guess, it's what everyone else uses. They're pretty cheap nowadays, grab a 250GB for £20/$25 and you'll never run out of space.
Not sure what you mean about change download directory? If you have a USB drive plugged in with available space, I believe it always uses that in place of the internal, that's what mine does anyway, I have literally no apps on my internal, I even moved the Youtube/Netflix/Lovefilm apps over to my USB HDD.







