I know it’s not official fluff, but I like the idea of there being a Necron tomb in the ruins of the original inhabitants of Necromunda, in the deepest parts under the hive. An Admech Exploratory Team searching for those ruins would explain their presence on Necromunda.
If talking GW official terrain, there is plenty to choose from, but the terrain fits your killteam's style, it's even better. So, it depends if your Killteam is one aggressively deployed against the AdMech's many enemies as an expeditionary force, a reclamation team hunting relics (or heretics) on lost, abadoned worlds or on defensive duties.
Expeditionary Force: would be fun to mix Death world fauna kits with Xenos terrain kits (Eldar & Tau terrain, or a destroyed Necron Monolith, or even an Orc Mekshop...) - contrasting the different technological design philosophies with your Killteam.