The newer (crio) slug barrels and the field barrel are the same outside diameter, so a different forearm would not be needed. If you purchased a used ER Shaw slug barrel, then you would need a different forearm as it has a greater outside diameter.
As for which way to go, I would call customer svc and see what prices are and what is available.
If you buy the slug gun, you would need the barrel and chokes. If you go the other route, you would only need the rifled barrel. If you need it now, are both options in stock? If you want to buy the barrel used, it would probably be easier to find th