alt Hacker NewsM3 Ultra with DGX Spark is right now what M5 Ultra will be in who knows when. You can just buy those two, connect them together using Exo and have M5 Ultra performance/memory right away. Who knows what M5 Ultra will cost given RAM/SSD price explosion?
I have researched a bit more and think your recommendations are spot on. The 256 GB M3 Ultra is probably the best value right now even though it's 2k EUR more expensive than the 96 GB version.