-yup.. pretty much.. I'd say getting a decent interface and monitors should be priority no.1 because you simply won't really hear what you're doing otherwise. Getting any kind of polished sound out of an inaccurate system is pretty much impossible.
-nope. For a whole band they can be handy if you're dealing with lots and lots of tracks. Even then, it's a terrible investment for a beginner since it really only duplicates functionality of your DAW and gives you tactile interface (buttons and sliders). That just feels nicer to work with, it won't give you new possibilities. When you're still building your kit, I would focus my investments on getting things that actually open up new possibilities.
-I'd go with Massive in your case. The Volca is super cool, but it's a kind of retro cool. It's a limited, almost toy-like machine. That's the point of it.
For actually learning synths and what they can do, you're much better off with one of the common virtual synths like Massive, Serum, Absynth etc.
The nice thing with these is that the workflow is very visual and immediate, and they offer a lot of different possibilities.. Massive and Absynth combine
a lot of different synthesis types like (virtual) analog, wavetables. Massive is the easier one to start with, Absynth is a bit more expansive and experimental.
Serum is pretty similar to Massive in my mind.. but it has some more options in terms of effects you can add which makes it another great synth to get your feet wet.
They all sound great, there's a reason everyone uses these
In general, I'd stay away from getting hardware at all until you have a good audio interface that allows you to work and record with low latencies.. any hardware you
connect won't run in sync otherwise and you'll have to look at things like running external clocks, etc..
Just overall any hardware you add is gonna add a few more layers of complexity and things to keep an eye on in your setup. If you like tinkering with that kind of stuff,
then it's great. If you're just trying to find out what's what so you can start making tracks.. it'll just be frustrating and you should keep things as streamlined and simple as possible.
Making music isn't complex until you make it so.