I have that question as well.
I own a CBox. At the time, I could but that and 2 240GB SSDs for less than I had paid for 2 128GB CFast 2.0 cards.
The cost of the breakout cables, at the time was $85 each. The rest of the parts totaled up to about $250. It made more sense for me, to go with the CBox. The time spend building it would have be worth much more than $250.
It has proven itself to be an invaluable tool for use with my Ursa. I highly recommend them, but anything to avoid the high cost of CFast 2.0 is worth it in my book.
I love my Blackmagic Ursa. It's the only upgradeable camera in that price range, so there's your future proofing.
I'm also a Sony fan, owning an FS100. I love that camera too, so something from their lineup would also be something I'd check out.
I purchased a CBox which allows me to use SSDs via the CFast card slots on my Ursa. It was an additional cost up front ($500, or you can hack one yourself for about $250), but has already proven to be an invaluable tool when working with this camera.
If your budget is $4000 for camera and lenses, the Ursa Mini is going to be out of your budget. A lot of people fail to realize the add on costs required to get the camera up and running.
For instance, CFast cards are currently $300+ for 128gb - you'll need two of those. Batteries and a charger? That's another $1000 for 2 small batteries and a charger. There's more you'll need too, but those are the biggies people don't often consider.
Find yourself a nice used FS100 and grab a set of Rokinon Cine Prime lenses. Unless you have clients demanding 4K, you don't need it.