Choice NI equipment for many(512) channels measurement

Hello! I need some advice about NI PXI or compactDaq.
Our laboratory testing electronic chips. We want by some new equipment. Our manager want to buy system with 512 channels to check big FPGA chip. But for me use PXI for this task is no logical. We do not need high speed measurement, we can test it consistently step by step, but how ? Use some multiplexor module its a bad idea, because their have own resistance. Maybe we do not now something about PXI or compactDAQ?  We need to by 4 module with 128 channels each? If we not need tested all channels for one time maybe we can do it use 32 channels and switch their ? How NI equipment can work with so many channels with low speed ?