I really want to invest in gear, but I'm concerned that it will become an expensive hobby.
Maybe yes, comparatively expensive, but compared to what? Web development is ultra-cheap, compared to everything else. Apart from computer and software there are hardly any other expenses. In sound you will eventually have to invest in microphones, recording equipment, and sometimes niche software. And even then, it's still cheap, compared to what, say, video people need to invest in. When you get to the point of having a place to do this in, that's when things can become more expensive than not.
Myself, I'm taking purchases slowly and thinking twice before each one. I've had the luck to see what's used by the pros very early. I believe it is possible to end up with all the essential gear even on a really tight budget. It won't cover all the tasks, but it's not my aim anyway. The same brand names come up over and over again, but a) there are alternatives and b) successful designs tend to get imitated, replicated, or plain stolen by vendors willing to offer them at a lower price. Then there's that specific client mindset, so often one would be wise not to say what they use :)
In any case it is essential to think of gear as an investment. some will last, some won't. Least you can do is have it pay for itself before it's out of service.
I think of time as the most expensive element. I mean, 10+ years! So any journey, any idle time, any downtime, becomes valuable: reading is high on the list of activities, so is learning and experimenting with the tools, building and organising a library, and so on. In my book, knowledge and flexible thinking are the superpowers for this activity. For everything else there's a way around.