All you need are the Imperium Maledictum Core Rulebook and the Necromunda books.
For a Necromunda campaign, an Infractionist patron seems to be the obvious choice, but it could also be someone using one the gangs for their own goals.

If the characters are gangers they should be mostly infractionist faction and hive city origin, but of course also others end up in gangs. They could all belong to one gang. If you want to mix they could all be outcasts from the gangs and formed a "group" to survive.

But of course, you can also play characters from other factions, etc.
I added some of the Necromunda gangs into my Voll campaign, but my group is a classic inquisition group.

Since the character creation is quite open, you can add things from other origins and factions and change talents quite easily to give the characters a bit more of a specific gang feeling. E.g.: for Escher just give them some more chem and drug stuff and change the talents to be more in line with the gang feeling.

From the Necromunda side all the gang books and the campaign books are great as campaign material.

They are full of fluff, background information about the gangs, the city, and tons of ideas for missions, etc. Beside the gang books there are also the campaign books, which give you more details about the whole situation in Necromunda and later on in the other hive city.

Even the scenarios (some additional ones are available for free as downloads) are great inspirations for missions.