WHATEVER YOU DO dont loose the stereo bracket
yes the bracket will fit a single din unit, you can use the plastic din tray to cover up the gap or go for a double din unit.
speakers are a weird size but you can still fit 6.5inch speakers from memory, thats what i have i think cant remeber now. you need to use spacers for them to fit and need to trim the door trim to fit them
if i was to do it again i would try and keep the door trim intact and fit the stock sized speakers
tweeters are in the doors.
dont bother replacign the speakers in the back they are not going to make much difference.
as for addign a sub do a search there a few different ways to do it
cheers