Software needs space to install, then you have to leave some free space on the system disk for the computer to work... Also, the bigger the drive, the longer it should last.
I went for 1TB internal ssd and I added an external Thunderbolt drive for sound libraries, recordings, etc.
I plan to fill the internal drive up to about 450-500 GB.
That's the space used on my Window machine, which I'm replacing with a Mac Mini. Libraries, recordings and so on are not on the system drive on that, too.
Actually, when deciding the specs of my Mac Mini, I went through the folders of my Windows DAW (system drive, applications, libraries I wanted to keep, projects...), I wrote down their name and their size on a notepad, then I summed them up to decide the internal storage and how much external space I needed.
I have 512 GB on my Macbook Air M1 (which I use for general productivity), but it feels a bit limited (currently it's warning me to free up some space); that's another reason why I went for 1TB. My previous 2012 Intel Macbook Pro was upgraded (by myself) to 1TB as well and in 2018 I did the same on my 2011 Mac Mini (which unfortunately died in 2020). For me, 1TB for the system drive has been the sweet spot (plus various additional internal / external drives, depending on the computer).
If you can afford it, I'd suggest to go for 1TB of internal storage... but then it's up to you and how you use your computer, what software you will install, maybe 512 GB are enough for you, I don't know...
AFAIK, you can boot you Mac from an external (Thunderbolt?) storage, so you could get a Mac with a lower internal size and then buy an external drive. In order to get the same speed of the internal drive, you need a Thunderbolt 4 drive, which is not cheap (I think there are not many options avaible). I have a Thunderbolt 3 external drive, it has enough bandwidth for my needs but it's slower than the internal drive of my Mac Mini M4. I may buy a newer, faster external enclusure down the line, though (I bought a Thunderbolt 3 enclosure because the price wasn't too much expensive during last BF... but it was still quite expensive, compared to USB-C).
A Thunderbolt drive is still cheaper than paying the so-called "Apple Tax", but won't be not "low price"...
Just for reference, I spent about 80β¬Β for a TB3 enclosure and about 270β¬ for a 4TB NVMe drive. And I had to buy a Thunderbolt cable because the one included with the TB3 enclosure was extremely short (I think it was about 30-40β¬, probably closer to 40β¬). And I took advantage of the BF sales, otherwise I would have spent more.
That's my personal experience when it comes to deciding the storage specs; of course your needs may be different.
I can't comment on the performance of the computer, I have not finished yet setting up all the software, but I can say it's very silent. I wait to put it under stress to see how it actually performs.