I'm still new(ish) to the game and have been doing sorties -- I have a forma'd bow (Rakta Cernos), Rifle (SomaP and BoltorP), Shotgun (Sancti Tigris), Pistol (Sonicor) but still need a Sniper Rifle (and melee but that'll probably be my NikanaP)

Which sniper rifle makes the most sense to dedicate my limited forma to for sniper rifle only sorties missions? I do not have Bladed Rounds OR Argon Scope. I have both Vectis Prime and Lanka built. All the Vectis Prime (which I'd prefer to use) builds I see on warframe builder use Primed Chamber (which obviously, I do not have.)

Which one and how would you build it without super expensive mods?