First things first, I'd buy Tomb of Abysthor. It is the best dungeon type module Necromancer Games has released - the environments are varied and interesting, and the scope of the adventure is just right. It is large enough to feel expansive but not overwhelming. The various NPC groups have an interesting dynamic, and may be dealt with in several ways from cooperation to simple hack & slash. It took me and my players about 60-70 hours to finish and we all agreed that it was great fun.
Raise the Dead is often overlooked, but the adventures inside are among the best Necro has published - these are all short "quest" type mini-modules in the case one party member really has to be resurrected... and important enough to - in true Necro fashion - risking even more death and dismemberment.
On the same note, Demons&Devils and Vampires&Liches are both collections of three deadly high level modules. I prefer the first one, but the second is also playable.
Finally, Ancient Kingdoms: Mesopotamia is just right if you are looking for a loose, exotic mini-campaign. I believe it comes close to Necropolis in imagination, plus it is much easier to use in a game.