There's some posts over on bmw5.co.uk from those who've done these kinds of retrofits and, I will assume here and on other BMW forums as well. It's up to you if you wish to go this way, but from the value for money, Time and effort, costs perspective it's not worth doing in my opinion. You'll need to change your instrument cluster, perhaps buy specific looms and you'll need coding done. Then, you'll need to find working MID units with pixels intact and from the correct model years to be compatible with your very late build E39. I'm quite certain you'll need looms to make those fit and quite possibly once everything is connected and installed the whole shebang will also need coding. You'll then need to find a working DSP amplifier and then retrofit all the DSP speakers.
You'll end up with a few more computer functions and oem - read mediocre, sound quality and the cost is likely to run into the hundreds of dollars range and I daresay not far off $1K.