I was wondering if it is enough with one M.2 500GB or if I should get an HDD? why? why not?
you should get as much storage as you think you need. however, it is very easy to add more in the future if you end up needing more than you originally purchased. if you can only afford one drive currently then you should get a solid state drive no matter what
Well this is a subjective question, and it really depends on the user, workload and use case. My development workstation has a 500GB SATA SSD, and a 500GB HDD and I never touch the 500GB HDD, it's really just left over from when the dept. upgraded from HDDs to SSDs. So that's fine.
At home on my gaming machine, 500GB is not enough. I mean I could work around it if I had to. But it would be inconvenient. At a minimum I would argue for a 500GB SSD either SATA or NVMe, and a HDD. Ideally, given the ever lowering prices of SSDs I think most people would be happier with 1TB+ SSDs and some sort of HDD. And for ultimate happiness, the way I've chosen to go, ditch HDDs completely and get as much SSD space as you need. Perhaps a really good SSD for the OS drive, and a larger budget SSD for additional storage. After all even a fairly mediocre SSD is going to be loads better than an HDD.
There really isn't a wrong answer though. It just depends on where your budget and your values lay. For some people storage is a part of the PC that doesn't really matter so they're only willing to spend $50 on it. Which is fine, you get what you pay for. If you value the performance and benefits a SSD gets you, maybe you decide not to ignore spending a reasonable amount on some performance storage.