Power for the backplane is delivered via four 4-pin molex connectors. The data connectors are four SFF-8643 ports.
I purchased a used LSI 9300-16i for the HBA (which has SFF-8643 ports also), so that I’d have the simplest cabling and not have to deal with a second HBA or expander. I could have gone with a 92xx series solution, as I currently just have spinning SATA disks and don’t need 12 gbps, but will probably add SAS disks and SSDs in the future.
I’m running TrueNAS Scale with a few VMs on the HL15. I realize proxmox is more powerful for virtualization, but I’m more comfortable in TrueNAS right now and my use case was storage-first/vm-second. I’m learning proxmox on a different system.
The backplane and backplane w/ PSU chassis options do come with fans installed. Specifically CoolerGuys 120X120X25MM 3-Pin Medium Speed. You can review this thread for all about fans and fan options.
I’m not sure the Noctua D15 will fit in the case, at least not with 140mm fans. See here. Other than that your build seems fine to me. For 10g are you planning to use SFP+/DAC or RJ45 connectors and Cat6 cable? I think 10G networking has become pretty much commodity now. TrueNAS/Linux should support most NICs.