Yes, M.2 works like any other HD device, except it has its own device name (nvme*).

Memory is up to you really. More a question of money.

With prices starting to drop on the second series Ryzen due to the next lot due soon, why not wait just that little bit more? No sense staying with the first-gen if you can help it. Will keep you going for a lot longer.

M.2: It's a form factor. There is a catch to it, though. It uses one of two protocols for communication; SATA or NVME. NVME is superior, but costs a bit more. Just make sure your M2 SSD supports NVME and your golden.

There aren't many, but there are still a few SATA M2 SSDs out there and I wouldn't want you to be disappointed.

In my 30 years of computing as a hobby, I think installing an M2 NVME drive was the single biggest performance boost I've seen. My rig boots OpenSuSe with the full KDE stack from cold in less than 20sec (and that includes me typing in my password).

Ps, that's a nice rig you have planned there. Mine is pretty similar and I'm really enjoying it.