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 Warlord1) 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 Kumanjaro1) 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 Necromancer1) 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.
ConclusionI 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.