Re:Speakers
The obvious place is to to put two in the front in the footwell pockets down where the bonnet release handles are. These are standard sized speakers and replacement should be very straight forward. A good quality pair should suffice IMO, but its a personal choice I guess.
Most 4 speaker set ups I have seen have the rears mounted in the rear "door cards" but that means cutting holes in the trim, not sure if you want to go down that route I wouldn't myself but thats just me. You have to be careful of placement because there are bracing cross members that can foul the rear of the speaker. (I've seen those cross braces simply cut out
Can't give any advice on iPod stereo's as I have a period radio fitted to mine! I know that you can get period stereos modified so that they can take an iPod connection so you get the best of both worlds. (Seen those at about circa £150)