Honestly, there's really no rule for how these guys have to look. In fact, I think it makes more sense to convert them, since they're supposed to be specialists that each Inquisitor has drawn into service... no two should be alike, really.
Fluff-wise, for instance, my Inquisitor's shooty retinue is drawn from survivors of "incursion events", mainly Imperial Guardsmen. So my gun-servitors are going to be heavy-weapons platforms (those dope Forgeworld Death Korps models) and my two Sages and one Mystic are the "spotters" -- the other guys on the platforms. The mystic's going to have some fancy-looking sensor gear, while the other two have the binocs and generally look like they're directing fire.
GW has a little section in Inquisitorial retinues as well, with some interesting conversions, which you can check out
here.
Have fun =)
-H
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-
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