1. The Chromebox has 16gb of internal storage. I'm fairly certain it's using the same SSD model the Cr-48s and other Chromebooks used, which is not easily replaceable (difficult to find a fitting card)
2. Chrome laptops and (now) desktops use verified boot, you won't be able to install your own OS unless you literally crack open the case (there are physical implements to prevent firmware flashing) and flash your own BIOS.
Just a note: there are three levels of access in the CR-48: regular (out of the box), dev mode (flip the switch), unprotected (open it up, take out the mobo, tape over a contact to disable bios protection).
The guides that don't require flashing the BIOS are nightmares. Awful, awful hackish nightmares. I say this as someone who did it. It's much easier to open it up, but a piece of tape over some contact, flash the BIOS and install whatever you like.
However, I flat out don't think the disable BIOS protection option is available on retail devices. (I have the cr-48).
If you want a lightweight Linux box, don't buy this, you don't want it, honest.
Shame really, you'd think that if you were producing a device that ran linux anyway, you'd be able to pick up a few extra sales by letting enthusiasts do their own thing with it. Since you're not paying Microsoft anything you've got a bit of wiggle room to be competitive pricewise too. (At the very least you'd think that Google employees might like something to run Goobuntu on).
It's all about trusting the integrity of the machine. With the signed BIOS, and the trust chain of ChromeOS, you can be quite sure that you're booting what you expect you're booting.
So, also from my point of view the OS provided in the beginning is not interesting.