What Lugia said.
The Z8000 sequencer is huge and the outputs are unquantized. Perhaps look into an Ornaments & Crime 8HP multi-function module. The price is about $250US and you get two 16-note sequencers that won't require a quantizer. It also has a dual quantizer function in it as well. If you pair that with an Expert Sleepers Disting Mk4, you get a lot of functionality in a little space. Some functions overlap, which can be an advantage. You can always add the Z8000 to a larger version of this system and your O_C and Disting will find other uses... they won't become redundant.
Look into the 1010 Music Bitbox if you're going to have on-board percussion. It's 26HP if I remember correctly and $600US. But you won't need all of those sample-playback percussion modules and won't need any additional modules to mix them. It also has easy loading alternate firmwares (free!) as a synth engine or an FX box. You can also record and loop any audio signals from inside your modular (it doesn't record DC). This will let you record the output of the RainMaker if you have some sort of awesome ambiance going and want to buy-back some of your modules that went into making it.
A clock source or divider is also a must (IMHO) for starting a system. Pamela's New Workout or a Temps Utile would work out well.
Also, the Rainmaker and Erica DSP have stereo outputs. You'll want some sort of mixer to take your mono signals and stereo signals and combine them... the same for any stereo samples.
And last, the Batumi has an expander called the Poti. It's 3HP. It will give you a lot more functionality out of the Batumi in terms of waveforms and sync/reset. You HAVE to have it IMHO.