SREAMING???

The long Ethernet cable will not be a problem.  It can be up to around 100 ft. Mine is about 50 feet too.

I believe the sony you mentioned is limited to 2TB on the internal HD there is another thread that discusses this where some have swapped out the 1TB drive for an ssd. 

"1) All-in-one unit with ripper or rip from PC? ( Only connection to computer is a 50’ cat 6 ethernet cable)

yes, but it depends on the cable and its ability and limitations. there are 2 camps in this debate: shielded or unshielded? which is better? best value? both camps claim victory, but I would not use an unshielded ethernet if it were susceptible to EMi/RFI- interferences.

2) Will this long cable cause problems with streaming and/or ripping?

see above. I have researched ethernet cables to run from my computer to my ethernet Renderer and also from the NAS to the DAC. For lower cost, entry level, look at the Supra brand from Sweden, probably the best bang for the buck available with very good results. Moving up a notch is the Wireworld Chroma ethernet, then up to the Starlight ethernet. Both of these are "Cat 8". Again, there is a web debate on Cat 6 versus Cat 8, etc. While i;m not convinced that a Cat 8 is signficantly better than a Cat 6, I just bought a Wireworld Starlight Cat 8 for my NAS to DAC for about $200. It might be a little overkill, but I wanted to make sure I got a good signal, b/c I won’t upgrade this ethernet cable for at least 5 yEARS, so I kind of wanted to "future-proof" it.

3) Is an external hard drive OK for storing music?

Yes, in fact in many cases it is the preferred method: a dedicated NAS: Network Attached Storage device. you will need a NAS with ethernet connectivity. Connect it to your router and will have a power input. This is where Roon comes into play, see below.

4) Is Tidal the only streaming service that streams in true Hi Rez?

yes and no, it depends on the type of music you listen to. Tidal, AFAIK, is the one with most modern music options- I use it. but there are other hi res Apps coming out, QuBuzz or something is coming out, but I think it might be more classical. Investigate.

5) Can you download music that is streamed and store it in the streaming unit or external hard drive?

You can with Tidal, and you can download from Tidal onto your phone, or tablet too.
Interested in the Sony HAPZ1ES. It seems like a pretty solid piece. (Has 1tb of internal storage but would need more eventually - at least 4 to 6 tb). That is a lot of storage, but many people are in your camp, the HAPZies should allow an NAS to attach to the back of the unit, unless Sony has restricted you to do this in order to push you to buy a more expensive unit with more storage.

.  Also very interested in Roon But not sure how it works or what I need to run that system."

I would recommend that you try Roon for a couple months. Roon and Tidal go hand in hand like peas and carrots (thank you Forrest Gump). Tidal provides the music but it is a "streamed" technology, not wireless.

But Roon does provide wireless service. So, if you have both, you can listen to Tidal on your computer, phone, tablet, and Roon will have an App on your devices, which you use to send the Tidal music over the air to your stereo. If you use the NAS attached to your router, I dont believe that you will need Roon for your main listening room. But Roon allows you to play music with different audio systems in the house wirelessly. Eg, in the kitchen or outside patio.

hope this helps, I have just regurgitated the things i have researched in the last few months for my own system!
Best