How much is a kWh in your parts? Noise, ambient temperature? You can buy very decent refurbished Lenovo tiny PCs with some 16 GB RAM and 6 cores and half a TB SSD which will run Proxmox and are low power and noise. You can go multi-node Proxmox later if you want to expand. k8s and related are also an option.

DIY few kWp solar payback is about 2 years.

In Germany consumer power is something like 0.4 EUR/kWh, so economics of running power-hungry hardware might be different. Solar PV might change the equation once again.