Hello,
This sounds like a very interesting project.
Here is what other potential developers will not tell you or do not know:
There is not a way to use the Google Map tile images (whether street view panorama or standard overhead map view) in an offline mode.
Some may try to download all the possible tiles and store them locally but that is against the Terms of Service from Google plus the number of tiles possible even at a municipal-level is far too high to capture for offline use.
There is an active request topic wanting an offline feature added to the API, but it simply does not exist at this time.
I have included pertinent links below. Anyone who tells you otherwise is not honest, misinformed or breaking the law.
Now with that being said, it is possible to use your own 360-degree image data and load it into the Maps engine for equivalent use.
It is possible to at least make a proof-of-concept for your application this way while waiting for an offline feature to be released by Google.
I have over 10 years' experience working with the Maps API, including custom overlays (map or image), heat maps, markers, info window manipulation, custom tools such as polygon drawing and measuring, & expert use of other Map APIs (Places, Routes, Geocoding, Autocomplete).
Pricing depends on using pay-as-you-go or premium plans, the former includes a $200/mo credit.
I have more info, but out of room.