Next to Mordheim, Necromunda is easily one of my favorite strategy games. As a TTRPG there is a whole wealth of possibilities, especially considering the rich lore and setting. A few years back I ran a small Necromunda campaign using the rules set from Only War. Since the core setting is Warhammer 40k and based around the Imperial Guard, conversions are not too hard. Here are my thoughts about what you need to change or adapt in order to get the game going:
Creating a character
Before getting into the mechanics, the whole group will have to agree on a House from which to build (this will replace the regiment option from Only War). There are several to choose from in the core setting, but I’m going to stick with the six original Houses. Also for the sake of ease, I’m going to abbreviate characteristics where possible. For those not in the know, they are: Weapon Skill (WS), Ballistic Skill (BS), Strength (S), Toughness (T), Agility (Ag), Intelligence (Int), Perception (Per), Willpower (WP), Fellowship (Fel)
Choosing a House
When picking a House, you will receive a bonus to three skills, and a negative to one:
Orlock: +3 BS, +3 T, +3 Fel, -3 Ag
Cawdor: +3 T, +3 WP, +3 Fel, -3 BS
Delaque: +3 Ag, +3 Int, +3 Per, -3 Fel
Escher: +3 WS, +3 Ag, +3 WP, -3 S
Goliath: +3 WS, +3 S, +3, -3 Int
Van Saar: +3 Int, +3 Per, +3 WP, -3 WS
Skills & Talents
You will also receive the following skills and Talents:
Orlock: [Skills] Athletics, Awareness, Command [Talents] Combat Formation, Rapid Reload, Sound Constitution, Enemy (Delaque)
Cawdor: [Skills] Common Lore (Imperium), Interrogate, Scrutiny [Talents] Cold Hearted, Sure Strike, Resistance (Fear)
Delaque: [Skills] Deceive, Security, Stealth [Talents] Double Team, Heightened Senses (Hearing), Streetfighting, Enemy (Orlock)
Escher: [Skills] Acrobatics, Dodge, Parry [Talents] Disarm, Lightning Reflexes, Iron Jaw, Enemy (Golaith)
Goliath: [Skills] Athletics, Intimidate, Survival [Talents] Berserk Charge, Diehard, Unarmed Warrior, Enemy (Escher)
Van Saar: [Skills] Medicae, Security, Tech-Use [Talents] Quick Draw, Technical Knock, Weapon Tech
From here you are limited to the following specialties: Heavy Gunner, Medic, Sergeant, and Weapon Specialist. There can be only one Seargent (who becomes the leader of the group), two Heavy gunners, and one Medic allowed in any gang. Weapon Specialist will essentially fill the ranks of any gang.
For anything that says regimental favored weapon, see the following list:
Orlock: [Standard] Boltgun [Heavy] Heavy Bolter
Cawdor: [Standard] Flamer [Heavy] Heavy Flamer
Delaque: [Standard] Long Las [Heavy] Lascannon
Escher: [Standard] Meltagun [Heavy] Multi-Melta
Goliath: [Standard] Grenade Launcher [Heavy] Missile Launcher
Van Saar: [Standard] Plasmagun [Heavy] Plasma Cannon
Essentially the rest of character creation is the same. Spend your extra XP, make sure your equipment is correct and you will be on your way.
The whole crux of Necromunda centers around making a name for yourselves, and to earn money/credits. I would advise using the price chart in the original war game (1995) for a baseline, or the monetary system as outlined in Dark Heresy.
Using the same idea from above, you may also want to add the option of having territories for extra income. Use the territories chart from the original game, or just forego it to make a campaign where income isn’t assured.
Now the rest is up to you! Pick a name for your group and start wandering the Underhive!