Here's the complete list of changes from previous editions to the 4th edition (when the Arena rebranding happened).

- New Art on spellbooks, game board, rulebook, box, and several cards
- Game Board folds in a different way so as to fit in the new, more compact box
- Some new tokens added. Previously they just said to use whatever you had at hand for tokens (different from markers which were always provided) but they got a lot of complaints that tokens weren't included, so they added them. These include wrath tokens, rage tokens, melee +1 tokens, and several others.
- 14 additional cards. These are: +1 Dragonscale Hauberk, +1 Feral Bobcat, +1 Leather Boots, +1 Leather Gloves, +1 Minor Heal, +1 Regrowth Belt, +1 Teleport, +1 Thunderift Falcon, +2 Dispel, +2 Dissolve, +2 Force Push.

In addition, like every previous edition, all of the cards have the most recent errata baked into them at the time of the printing. You can find all errata for all of the cards in the Official Rules and Codex Supplement. (also, when I say all of the cards, very few have actually received errata).

I believe I already talked to you about the differences in getting a 2nd core set and getting 3 spell tomes here on boardgamegeek. Thus I'll move right along to your second question.

Forcemaster vs Warlord
1) Provides two new mages from two schools of training that none of the core mages have. These mages provide very different opportunities for game play.
2) New spells are mostly focused around the afformentioned schools (Mind, War, and Earth) which balances out the spells available.
3) Provides support for an Earth Wizard build, but otherwise lacks much support for the core mages.

Conquest of Kumanjaro
1) Provides great support for all of the mages.
2) Cheapest of the three expansions.
3) Fewest spells of the three expansions.
4) Provides 2 new mages, but the premade spellbooks can not be built with just this expansion and the core set. You need additional products. You could choose to ignore the premade spellbooks however and just build custom books. However, even if you build custom books, you'll have a hard time building these mages plus the original Priestess and Beastmaster, which means these are really more replacement mages than new mages.

Druid vs Necromancer
1) Provides two new mages which provide very different opportunities for game play. These mages have some overlap with core mages, with one being another nature mage and the other being another dark mage.
2) The new spells can be quite useful for the core mages as well as the new mages (at least the Beastmaster and Warlock). Although this overlap might cause some competition over spells.

Conclusion
I think you would get the most bang from your buck out of either Forcemaster vs Warlord or Druid vs Necromancer. Don't get me wrong, Conquest of Kumanjaro is great, but it doesn't add as much variety to the game as the two bigger expansions do. Between the two bigger expansions, I think they're both good and bring a lot of interesting things to the table. Methinks you'd be happy with either one, so pick the one that most intrigues you.

I would personally get them in the following order: Druid vs Necromancer, Forcemaster vs Warlord, Conquest of Kumanjaro. That's mostly because I like the theme of the Druid and Necromancer better than the Forcemaster and Warlord though, and I could easily be persuaded to reverse their order.