It'll affect your speedo. Your car could be telling you that you're going 100kmh, but you could actually be going at about ~103kmh. You could get defected for them aswell.
Changing springs isn't all that hard. You can do it all yourself in about 2hours tops if you know what you're doing. There are quite a few tutorials on how to do the change.
I wouldn't recommend changing the springs and leaving the standard shocks in there. Your ride is going to feel and ride like shit.
I think that will mostly be cosmetic things like seats, windows etc. You can still use the coupe suspensions as far as I know. I don't think there's much difference in handling either except the sedans weigh a bit more and might affect the braking distance for example.
I think they just put it at the bottom to hide the washer. Having it at the top means having the wiper at the top too and that'll look gay in my opinion.
Just use the offset calculator. Put in the specs of the wheels you have on at the moment and the specs of the wheels you would want. It'll tell you how much it will stick out/in from what you've got now. You can decide from that. It's just a guide though, it won't tell you whether or not it'll rub
They are different in size, weight and looks only as far as I know. You can use the same suspension from the coupe on the sedan as long as they're the same model (gts-t to gts-t).
I'm not too sure which diff is better, but if you're wanting to drift, you're going to need an aftermarket diff or a locked/shimmed diff to be able to drift without any trouble (except maybe cops if you've got a locked one).
I just realised this morning that the 32's handbrake position is shit. You can't have your car in reverse and push the handbrake button down easily. Just say you are on a hill and need to reverse out. The handbrake is a bit close to the gearstick.
I think the only way to improve the reception is to get some sort of signal booster. My radio sounds like shit at times. Always goes good, bad, good bad when I'm driving.
You'll be able to get coupe skirts to fit your sedan, but it will involve a lot of modifications to the skirt. Best to buy a kit made for the sedan mate.