I would definitely recommend VI - even if you don't play the Viking Campaign, the new units, factions and the reinforcement set-up make MTW even better!!

But the Viking campaign itself is great (esp if you're a Brit, personally I'm a Mercian but I haven't played the faction yet....).

The battle pack is still going strong in the UK, I'm sure I saw it on some in-store "chart topper" racks just before Xmas, and of course Amazon still do it.

As for the mods, I haven't tried any yet, though I will have a shot at XL and the BKB one as soon as my current empire has swept away the infidel West... I'm amazed to find a game I still haven't exhausted after two years. Few last more than a couple of months before the fun wears off. And I've learned so much mediaeval history, as well!

Oh, I haven't got RTW yet as my machine is too low spec, won't even run the demo properly