if you get an 03 you will not have to change out headlights and tail lights (they are clear on 03), side vents are updated, glass window, rear bumper is very nice looking, so is a front one, but as far as what you want you can change it over and ebay the original to get some $ back. all these costto update, so might as well start fresh w everyting on there already. but then your only mod is bumper heheh.
but generally speaking w these cars, the newest you can get w lowest miles
all you then need is some suspension and wheels.
gt3 bumper will be a direct swap, front bumper comes off actually fairly easy, few minutes and its off, as long as you get the correcponding year bumper.
as for the headlights, if you want the newer style fenders and hood and other little things need to be changed, quite costly when including all the labor, paint, cost of parts and headlights themselves as well.
jeff on here has updated his boxster, look it up
nice e36 man
hope that helps clear some things up
ask away, we're sure you have more Q's