A7R3 is a good choice. Since you will be purchasing in a few days you need to ensure that the camera has the latest updates in both camera firmware and Lens software because new versions have been recently released. If you buy at a full service store, sweet talk them into doing it for you. If not, then you'll have to go to the Sony support site and get the instructions and download 2 pieces of software. If your MAC is 10.13 (High Sierra) or higher the first piece of software ("DriverLoader_1013") is required. If your Mac is 10.12 or earlier, then only the second piece of software is required. The instructions were hard for me to understand so I would suggest reading all the instructions with both pieces of software. I had to read them multiple times.
On to your question about a filing system. I use Adobe Lightroom for basic file management and editing. I also back up my RAW (in my case I use DNG files) in an external portable 5GB drive. I also back up edited DNGs in the cloud. I would recommend probably one more place.
Now, about workflow - in LIghtRoom you can import all the photos from your SD Card. Then you can use LightRoom to run thru them and pick the ones you want to keep. I then edit and update all the metadata and tags, in order for searching. LightRoom will offer A LOT of ways to organize your photos, so I'd recommend it as your main organizer. When I'm done selecting and editing, I then export jpgs of the DNG files for showing and upload them into my site. I back up everything including those I don't want to keep in my Google Drive (2TB for $10/mo). Due to your limited space, after you finish this workflow, you could remove your photos from LightRoom (and your computer). You'll have to find a workflow that you are comfortable with, so it will be a LOT of trial an error.
I hope this helps a bit. There are certainly many, many people much more experienced and skilled than I to offer you advice. I would suggest searching these questions on youTube. You'll find many helpful videos. Good Luck