Office build $700 budget - please advise

Regarding the supposedly missing parts, I was just using the custom price for the PCPP link in place of the Newegg combo buy price (not including the $20 rebate for the MB) for the MB (Gigabyte H110M-A) + CPU (i3 6100) + RAM (Kingston 1x8GB Fury 2133@ CL14) that I listed earlier in the post.

PCPP used to support combo buy price reduction in its listing but that feature seems to have been fallen by the way side (while promo code/coupon price reductions has become spotty; rebates is about the only price reduction that is still mostly functional on PCPP; with Amazon rebates sometime don't get counted).

The H110M-A has two DIMM slots and with the Kingston 1x8GB 2133 (the maximum bus supported by Skylake CPU + 100 Series MB), you'll still have a second DIMM slot open for another 8GB stick if/when you need it. The only niggling detail is that the module in this Newegg combo is the single rank HX421C14FB2/8 not the older dual-rank model HX421C14FB/8, as dual-rank modules would have had better performance when paired with Skylake/Kabylake CPU even if the bus/CL looks to be the same.

The 6100 is good enough to function even as a basic media transcoding server CPU. The H110 chipset on the MB however does not support RAID (it's only natively supported on more expensive MB with H or Z *70 chipset), so you'd need an expansion card or an external solution for backup/redundancy.

From the sound of what you're describing though, I'd say one external backup drive with a weekly backup schedule for all those doc files to that drive plus Dropbox, Google Drive or MS OneDrive, etc as a tertiary backup plan that will backup the files at a higher frequency than the external drive since they are more resilient to natural disaster damage than any solution that you can employ yourself. If that data is of utmost importance to you then remember: Backup, backup and verify. Don't forget the last part because backup can fail, the drive use to backup also isn't immune to failing. Storage devices can fail whenever wherever and for whatever reason (and when you buy online, you have practically no control over how it's handled before it reaches your hands); you certainly can get lucky (I've still had a working Maxtor 80GB UltraATA drive with > 10 years worth of operation hours on it) but don't bank on a storage device not dying on you.

Remember to make a backup of the SSD after you just finished installing Windows, Office and the associated updates to an external drive so that in the event that the SSD should fail, the system'd only be down for the duration of a new SSD delivery time (+time to reapply the updates that will have been issued since then).

Kabylake and the associated 200 Series chipsets (as well as AMD Ryzen platform) is not officially supported on anything but W10. Unless MS changed their stance while I wasn't looking, I believe that the main compatibility issue is that you'd lose access to updates if W7 realize you're using it with either Kabylake or AMD Ryzen and you really don't want a system connected to the Internet and your network to be without updates and open to vulnerability; which is why I went with a still supported Skylake config instead. The H110 board should ship with the updated BIOS to support Kabylake CPU if and when you want to as it's still the same LGA1151 socket; 8th gen Coffee/Cannon Lake should still be on this socket, but until it is released, BIOS support for 8th gen is not guaranteed. Even as someone who is on W8 and liking it, I'm not going to tell you to move on from W7 when you neither want nor have to.

Now, if you feel like you're ready and/or want to give W10 a spin, then going in right away with Kabylake/200 Series chipsets MB is going to be the better choice in the long run for support/better support of some newer features because performance-wise there isn't any difference between 6th gen Skylake vs 7th gen Kabylake (7th gen is just built upon a more optimized manufacturing process that allow Intel to push the chips to higher clock while clock-to-clock they're practically the same).