I built a GSC gang for my friend a few months back, and they are surprisingly competitive, but I feel like you have to play them in a very narrow, highly specific way. The Adept is a good psychic leader, the Cult gets cheap hand flamers, and all your champs & leaders can take a psychic familiar that makes them stupidly hard to kill. The Cult is fun to play if you've already got models lying around for Kill Team or whatever, but I feel like House Cawdor is thematically similar, and they would give you more bang for your buck.
If you want to read more about GSC in Necromunda, this Goonhammer guide is really great:
https://www.goonhammer.com/necromunday-houses-er-cults-of-the-underhive-genestealer-cults/
To answer your specific questions: the Magus & Primus are both usable as leaders. I'd go for Magus if you have to pick one because that's the best model to use for the psychic adept leader. I think you can also use the icon bearer by hacking together a few of the character options, but I don't think it's a very great build. Neophytes will be your bread and butter, so pick those up. I would skip acolytes entirely: third arms are too expensive to use more than maybe one, and heavy melee weapons are even less cost-efficient. Like I said up top, Psychic Familiars are arguably the best model in the Gang, and you can use up to three.
I hope this helps.