In case you have not noticed, he's talking about an L322 not a P38. The L322 is CAN BUS and totaly different to the P38. Forget serial to USB converters, not relevant. His best bet is Storey Wilsons new kit which has the hardware interface for CAN BUS to USB.
Not sure of the dates, Matt Stevenson would know, but there are changes as the years go by, I do know that the comms are totaly different to the P38 and a hardware interface is required. BBS do the kit but it's pricey for the L322 I think. Storey Wilson is the cheapest, if untried option.