John,
On a SP2 with 512 GB storage, I have about 435GB available. I don't have many other programs other than C1 Pro 7 DB, Photographers Ephemeris, and Malwarebytes. I have not "cleaned" the SP2 of the other surplusage programs, which would free up more space probably. I also have a 64GB Micro sd card as backup in the slot if I need more space; you can use up to 128GB card at the moment for even more space.
SP2 works great for me now, though the SP3 is tempting with its slightly larger screen. But it is the tablet form factor that makes tethering with the Surface Pro such a great option over laptops.
You really don't need "all the accessories" with the SP2---I leave mine off when tethering. Arkon clamp runs about $50 and a USB3 1.5 foot cord is about $8. The Surface Pro is the best upgrade and a helluva lot less expensive than an upgrade to the IQ2 with its very limited WiFi to Ipad capability. Surface Pro tethered option beats IQ2 WiFi hands-down, imho.
ken
p.s. USB3 thumbdrives make it easy to transfer session folders (raws) to your main workstation for processing, and also can serve as temporary storage if needed to free up space on your SP hard drive. I've seen relatively inexpensive USB thumbdrives with 256GB capacity, not quite Sandisk performance, but still pretty fast.