Cubase 11 will most likely be released by the end of the year. At this point, I would probably wait. Or, if you purchase 10.5 now you can wait to activate it after 11 is released and hopefully get a grace period update.
I think boxed versions might be a thing of the past at this point.
Many audio interfaces are available which would fall into that category. Your budget certainly will be a factor here. How many inputs do you need to record at the same time? Mics, DI instruments, line level signals? Do you need MIDI in/out? You can also expand input count by connecting another preamp unit with ADAT if the interface supports that. Do you need two headphone outputs? Balanced monitor outputs? What type of connection to the computer are you looking for? In Steinberg land I think something like this would cover a lot of ground …
It also includes Cubase AI. Depending on your needs, this might be all you would need to get started.
As far as installation/storage usage goes, if you have enough space on your current OS drive I don’t see any reason to not install the Cubase content there. The application itself should always be installed there. My Steinberg content lives on my OS drive. Then I have another drive for third party (Superior Drummer, SampleTank, etc.) content, and another drive for projects and their associated audio files. One more drive for general storage. This isn’t a necessity so much as an organizational thing for me. It all depends on what you are doing for projects, how much third party content you might invest in down the road, current drive space, etc.
For the other software, it’s hard to say what you might need. Until you need it! You’ll know it when the time comes. Start simple and work your way into it. Many times you can find good deals on software if you are not in a hurry. Keep an eye out for sales and you can save money for sure.
The only other thing I can think of right now would be a MIDI input device if you plan to use many virtual instruments. Something as simple as a KORG MicroKEY can make things a lot more fun. I have one of 37 keys and think it’s great …