I'd be inclined to give the infantry squads heavy weapons before taking any fast attack choices. Taking the maximum number of heavies and specials in infantry squads has served me very well so far. Against some enemies you double your ability to kill them by increasing the squad's cost by a mere 33% or so.
Your background might influence this however. Just how Celtic and "in your face" would these guys be, when bearing their background in mind? Are they complete berserkers or more reasonably minded?
Now that I've though of berserkers, that would be an awesome unit to use as Ogryn substitutes. A bunch of hairy, animal pelt wearing, unwashed frothing madmen with great weapons on Ogryn sized bases!
I'm nearly tempted to start a Celtic army over this idea. If you have the chance, check out the abhuman doctrine for feral Ogryns and see how you like it.
where to find itBut I digress. Because of their ties with the Adeptus Mechanicus, a considerable number of heavy weapons certainly wouldn't be out of the question, depending on how (un)civilised your Celts are.
@xsimplex: The laspistol & CCW is an option available even to normal guardsmen in command squads. No need to upgrade to veteran status to get access to said weapons.