It would be to have two NAB6 mini PCs, I have seen them on Amazon and they are good machines. To those two machines install Proxmox and make them high availability.

Yea, but they won't be High Availability unless you have 3. Proxmox assigns HA Dynamically, meaning the machines vote. So you need a odd number. BUT, a cluster of 2 should be fine. Migrate the VMs over, to power down and perform maintence.

And then to that add a box with Truenas for storage. This is what I see a bit more complicated. Can you advise me something to build that does not go too much price?

I really like the n5105 nas boards, and the jxxxxx nas boards. I went with n51505. Basically what "Wolfgang" did on YouTube. It's more than powerful enough, perhaps overkill. But the benefit is it's a lot of sata ports, on a mini it's board, which is rare. You might find a better setup, I'm still planning this section out.

Only with the miniPc the budget would go to 900€, I currently have a synology ds220+ (that I plan to sell), with 2 4TB hard drives.

You could just do what I did initially. I just copied Wolfgang's build. N5105 nas board, a nas case (I used the fractal node, in his video), and put 32gb of ram (overkill). And a psu with a 500gb nvme. You can install TrueNAS on here. Run everything on here, media server, containers, etc. set your drives to ZFS. Then when you want to expand, you can get the NAB6 or something different...

You need to plan for what you intend to do. The setup I recommended is for like many VMs, for home labbing enterprise software to learn it. In addition to home server stuff, like media, are, vault, cloud, immich, etc. even a few windows and Linux VMs. It's A LOT. You can simplify it. You can even run proxmox on the n5105. And just set up a ceph ZFS storage pool.