My take of the Alpha Legion is that they attempt to look like normal Marines rather than the spikes and tentacles of the other traitor legions. Their subversive activities mean they need to pass as normal marines.
We are marines from the Hydra Legion, now where is your central defence hub?Their equipment is going to be a mixture of old and new, picked up as they go along.
This is exemplified by this Terminator.
With the exception of the weaponry and the head it looks like a regular terminator suit (except that it lacks a Crux Terminatus). Armed with the old style Reaper auto cannon shorn of any overt chaos symbols it would appear to be a loyalist suit.
Built from a Terminator body with the head opening widened to take a Marine mark 7 helmet. The reaper is from the Chaos Marine Terminator kit, the power glove is a marine one as are the two shoulder pads. The Crux Terminatus was shaved off leaving the skull.
The body was sprayed with Citadel Incubi Darkness and I have started to paint the black base coats on the weapons, cables etc.