hello and welcome.
personally i would go for an external filter,there are a few on the market these days,
i think there have been a few threads made on here talking about them.
Eheim have always been a good make,i myself use Fluval,there are also Rena
which i have also used.
i wouldn't bother with an undergravel system at all,just use an external.
the carbon,is sometimes included in the media package that comes with the filter,
you only really need it if you are going to medicate the aquarium,or when you buy bog wood,
it takes the tea colur water away,i run carbon in my tanks for this reason,and i change it
once a month.
good luck with getting the tank,and do post pictures if you are able too.