There's just the 3 you can get at the start. They're good, but not always uniquely amazing. They're basically black-market versions of other weapons in game, slightly modified.
M-11 Wraith (250,000)
This is basically a better version of the Eviserator but it has slightly less capacity. Modest weight requirement, very high damage shotgun with otherwise average stats. It's not amazingly superior to the Eviserator, which is in comparison extremely easy to get.
M-77 Paladin (200,000)
A super light, super accurate, high damage pistol, this appears to be identical to the M-6 Carnifex. You can pick up the Carnifex for free during the mission Arre: Ex Cerberus Scientists so I have no idea why you'd buy this honestly.
Black Widow (250,000)
Slightly modified version of the M-98 Widow Sniper Rifle, this gun has higher capacity for more bullets between reloads, higher fire rate, slightly less damage per shot and it's slightly less accurate. The fire rate and capacity boosts seem to make this one worth it.
Specre Requisitions also gets a few other items if you miss armor/mods on missions, but there's nothing special about them. For example I just found the Rosenkov Materials Greaves there for 5000, they're only there because I missed them in a mission, they're not Spectre specials.