My main question is, is 512GB enough?
If you're doing pro video production or Big Data, nothing is enough
but that might mean that you have to rely on external storage anyway.
For more general use I'd say:
256GB is
ok for "personal productivity" use (email/WP/web browsing) especially if you keep data in the cloud and expect hot and cold running internet wherever you work - but install a couple of Pro apps, start messing around with virtual machines, or any sort of media creation and it's going to be tight & its a joke to have so little on anything in the Mac price range.
512GB is far more satisfactory if you have a few Pro apps, dabble in audio or regular compressed high def (i.e. maybe not high-bandwidth pro formats or 4k) or for coding etc. and don't want to carry around (say) a movie/TV show library. Be prepared to "curate" what you keep on your hard drive and offload less frequently used stuff to external drives. I think there's a good argument
on a desktop for having a super-fast 512GB system drive for systems and apps and keeping your data on external or networked storage. For one thing, Apple SSDs live and die with the machine they're embedded in and you may not want too many eggs in that basket. Also, unless you're editing multiple streams of 4k video or something, as long as the system, swap and temporary files are on the super-fast Apple SSD, slower, cheaper external (USB or networked) storage will likely be perfectly adequate for your "documents".
On a laptop, though, it's nice to be able to have everything in one place without faffing around with external storage.
1TB
should be the sensible minimum for any computer with "pro" in the name, but Apple prices and base specs make it difficult. I have a 1TB drive on my Studio and its about half full (not too bad since I've got XCode installed, mess around with Logic & recording audio, and have a couple of Linux VMs) but I do shove "archive" material off onto external storage from time to time. So I could cope with 512GB + external with a bit of discipline.