I have a 2004 Polo 9N, very basic (even has wind-up windows!) and was wondering if I could retrofit the MFD that the higher-spec models have?
Yes. However, it would probably be more trouble than it is worth.
If this was an option for your car, you just need to obtain all the related parts and install them. This would be at least the MFD and related wiring. Maybe the ECU, more wiring harnesses, sensors, etc. would be needed.
If it wasn't an option for your car, then quite a bit of DIY wiring would be involved.
Either way I doubt it would be easy.