I'm not convinced McGahee is a very good fit for the Manning offense. He's not much of a threat as a received out of the backfield. He's on the wrong side of 30 and coming off another knee injury. He seems to have developed an issue with ball security.
Moreno outplayed him after McGahee's injury and, from what I can tell, did fine as a pass blocker. Hillman's big issues are pass blocking and ball security. But that's a decent tandem to work with.
That said, if Jackson signs a very cap-friendly one-year deal for Denver I could see it working. He's not the RB he was 5 years ago and he's had durability issues his entire career. But he's a tough runner and an exceptional receiving back. So yeah, bring him in for a look and if he's cheap and short-term, why not?