Hi Mick, welcome to the site and a great game! I would mostly agree with these guys. I would ask you a couple of questions first though. Where do you live, and do you want to aquire them all at once or buy them a few at a time? My next question would be how far are you interested in going, meaning, do you think you'd like to make fan-made expansions some day in the near future?
Whaley is correct, the
Big Box 2 already comes with the base set,
Inns & Cathedrals,
Traders and Builders,
Princess & Dragon,
Abbey & Mayor, and
King, Count & Consort (which included the
Count of Carcassonne, the King half of
King and Scout,
River 2, and 5 shrine tiles), and I would start with that. I'm assuing you live in the U.S., is that right? Unless you would like to support your FLGS (friendly local game store), I'd definitely go with either
Fair Play Games or ebay. Ebay is about 2/3 the cost of my FLGS, some times less. They're friendly, but not that friendly.
(Though I did buy my initial sets there.)
I'd go Big Box,
Cult, Siege & Creativity (which comes with 6 shrines as opposed to 5 in the Big Box),
The Tower, GQ11, and
The Catapult. A lot of people don't like the Catapult, but there are some of us that do. I would suggest buying it simply for the unique tiles it has. There are some tile formations that don't exist in other sets. If you don't like the game mechanics, you can always go with Joff & Scott's
The Jester and the Minstrel fan-made expansion. It works with the Catapult tiles. I'd also suggest you buy another base set. I'd do this for two reasons. It saves you the hassle of having to find a stand-alone River 1 set and you'd have 72 tiles to use to either make fan-made expanions with (not to mentions extra meeples which can come in handy) or you could give the set away to a friend who you'd like to introduce the game too. I'll probably also buy the
Wheel of Fate/Fortune, whichever it is, when it comes out. It is basically a re-release of the original base set with a few different tiles and some new mechanics. You could just incorporate it with the rest, or take out the corresponding tiles from the regular base game set and use them for fan-mades. I think there are about 60 or so. The extra meeples in this set could also come in handy.
Fair Play Games is an excellent place to buy them becasue you can get everything on this list above for only $95.94 (pre-tax). If you can bump that up to $125 with other stuff from your wish-list, or other extra sets or spin-off games, you'll get free shipping. That would probably pay for itself. Hope you find this helpful.
