Oh man, it seems each time I look at GW's store there some of the old minis missing...
Anyway, I was thinking of the arm from the command squad, but that is a left arm and not a right arm. The old metal devestator box and the vet sarge from the old command squad came with this
arm, though. The only pointing right SM arm in the current range is probably from Chronus
. However, if you take a boltpistol arm, turn the hand a bit, cut the pistol and the trigger finger off and replace them with a bit of wire and GS or a bit of carved plastic you end up with something just as good.
Any skull with a bit of wire stuck in, would do as servo-skull. You can add other bits like lenses from targeters and wire as antennas etc. in too. There used to be some nice metal servo skull in the Inquisitor Henchemen blister, but GW doesn't sell these anymore. The only metal ones are now in Coteaz's box
. If you intend to get some Devestators or heavy weapons in general at one point, there are four plastic servo-skulls on the sprue as can be seen here
Even if it really is your "first major conversion", it should be pretty easy to do.