I would tell your customer that they need to upgrade the PC as part of the package. This will also give them a fall back position - should anything go wrong with the upgrade the existing system is unchanged.
It will also make your life easier as much of the work can be done at base rather than unknowns on site - potentially leading to savings for the customer as on site work and travelling is costly.
You can use USB to Serial adapters rather than having to use internal cards, this makes it a standard PC should anything fail in future. Easysync make industrial USB/serial convertors.