Will the full build HL15 support running VMs out of the box?

I agree to the earlier response.

Each individual may have a preferred hypervisor.

Some platforms are shipped with their own ISO while other software can be installed via the package system.

Just because 45 drives ships Rocky it does not limit you. This is a DIY system. All the hardware have manuals available (to understand the motherboard chipset, etc). Weblink below

You can install anything you prefer. Xcp-by and proxmox have a dedicated ISO to configure everything. You can also install QEMU independently (or can server).

You can go the TrueNas scale route. This is the nice thing with the platform, you can choose the path you want to take.

45 Drives is offering their Houston system for you to add if you use a different distro.

For me: I will probably keep the NVMe that is shipped with the unit as a base testing platform, but use my own NVMe to try other distro/configures.

Feel free to ask more or share what you are (or have done).