Glad to hear you picked up a rig! Terry West has great quality cables. I'm assuming you'll want power cables for your Venice, Arri 12v mini/amira and Arri 24v for arri film cameras/XT's- he'll be able to make you all of those and they'll be braided and at a fair price. I'm sure he can do 3pin to ptap breakout as well. Here's his cell (Text/call don't email) Terry West- 1 (310)-621-5063
For weight on the bottom, one thing you can do being you don't have the luxury of a third battery plate, is throw a shark fin on the second battery plate directly under the post once you get one. Not only will that give you the extra weight you need for most builds, it will sit the weight a few inches lower which will mean a lot when you need the drop time for for heavier setups.
As for where to get the yoke upgrade, second battery plate, or Zephyr specific posts, I would call Tiffen, and also hit up Tom Dean or one of the guys who have a lot of experience specifically with the Zephyr on instagram. They may have some insight. Happy hunting.