Many of us follow the "on budget" restriction, but I see the 4ms pod more as an auxiliary box, and not very comfortable to chain with others. In addition to the recommended TipTop mantis (6U 104 HP) you have some less ambitious options to start with. For example https://koma-elektronik.com/?product=koma-case-3u-84hp-powered which would cost you about €90 more than the 4MS. The good thing about it is the system to grow, you can easily add another 84Hp case using the same source, depending of course on the consumption of modules that you include. €90 is money in real life but nothing in the modular world, believe me.
Regarding the modules, I miss a quantized pitch source and some gate or clock generator.
A filter is essential too. Doepfer has good filters at very affordable prices.
The VCA and envelopes part you have covered at the moment with Maths.
A small mixer (perhaps also from Doepfer) will allow you to send more than one signal from your Dixie (it has 6 outputs) to the Z5000.
To start and adjust to the budget you can always look at modules from Ladik or Doepfer. They have many utilities at a very good price.