Putting large drivers in doors isn't a good idea for an ICE novice. Chances are it'll just rattle and be annoying - remember that astras rattle enough as they are. You'd need to be pretty competant with fiberglass/woodworking and be willing to invest in decent sound deadening (which it seems not many Mk3 owners are), your probably looking at £100 worth per door to get a 8" sub playing right.
Apart from the rattling, there's the space issue. You'd have to separate the air space between the sub and 6.5" driver - they're not designed to work in the same enclosure, and there just isn't room inside the mk3 doors to do that. Although a proper 3 way kit would probably be ok but these typically match a 6.5" mid-bass drive to a 4" mid, not a big sub-bass driver. And 3 ways are more expensive.
My recomendadtion would be to build some pods that offer enough clearance for a decent set of 6.5" components then get a sub for the boot. You'll be supprised how low a good 6.5" will go, and sub-bass is non-directional. If you didn't already know it was coming from the boot you'd have a hard time telling at sensible listening volumes.
It does get a lot more obvious where the sub is when you turn it up to 11 tho, especially if you haven't deadened the boot lid and it starts try to shake itself apart.