What you can do on a computer today is so much greater than it was 20 years ago that it would have been unimaginable back then. It is something a musician can be really excited about. The activity you have mentioned is composing symphonic material. I’m wondering whether you might be happier in the long term with Dorico rather than Cubase. Speaking only for myself, I think of Cubase as recording software, and Dorico as composing software.
If you are primarily interested in composing using notation and producing scores and printed music, I would suggest looking at Dorico, instead of Cubase. Yes, Cubase has (limited) scoring functionality, BUT it is quite primitive in comparison to Dorico. I doubt you would find Cubase conducive to this type of workflow.
You WILL need a hardware interface in any event, and buying a Steinberg interface seems like a good starting point, as it would include a free version of Cubase. If you go this path, be sure to get an interface with MIDI I/O so that you can use your MIDI keyboard. This would give you a quick picture of what Cubase is, though in doing anything approaching a symphonic composition you would find yourself quickly exceeding the available track count in the free version.
Dorico does provide a trial version, so you can try it out and see which approach would better suit your working style. There is a quite substantial learning curve for both of theses titles, so give yourself some time to make a fair comparison.