The PCI-6281 is a decent card, but it costs over $3,000 and is marked as Last Time Buy, which means NI is about to discontinue selling it. Using a PCI to ISA adaptor could also be an issue.
Have you considered a PCIe card. The PCIe-6320 costs $755 and exceeds the performance and specs. of the Lab PC+. With the leftover budget you can buy a new PC.
Whichever card you select, you will need to rewrite your application software to use the NI-DAQmx driver. Neither the 6281, or 6320 support the NI-DAQ driver used by the Lab PC+.
There may be something I don't know about your system requirements, but buying a card that has a list price of over $3,000 and is about to be discontinued is an interesting choice.
As for schematic drawings, register level programming manuals, etc. NI does not make those available. You can find connecter pinouts and basic functionality information in the documentation.
PCIe-6320 - NI
NI-DAQ™mx Download - NI
