Skip the PXI DMM, FG and scope cards, they are expensive and usually do not perform as well as standard instruments. Instead pick whatever instrument fits your requirement with a modern interface (LXI, USB) and a rack kit.
Example Keysight 33465A vs NI PXI DMM. The NI card is ~4000USD, with slightly worse performance.
Definitely use the PXI for massive multiplexers, DAQs, Digital interfaces (CAN, I2C etc) and GPIO. Also, take into account that PXI is being phased out in favor of PXIe. Some of NI PXI muxes are EOL, without an alternative.