You should use the GetNewPart method, and then check if that part has a Std Cost and if it is Qty Bearing (as Qty Bearing parts are the only ones that can go into stock). See the following thread that had a similar issue of wanting to check for a Std Cost before allowing the part to be added…
That thread only checks the StdCost (and see later post about adding CostID for extra data integrity). You could do a similar thing to check the parts QtyBearing status. Or check the PO line type (Buy to stock, job, or other)
If you are multi-site, this could be complicated by the fact that different sites can have different Qty Bearing and CostID’s.