AFAIK the most secure way to host stuff on your home network is to set up a locked down, firewalled VLAN for it.

Honestly that depends on the specific hardware and what your walls are made of! And the power consumption would also depend on your power plan, could be anywhere from a dollar to $5-10+ a day.

Shipping any server here from overseas would prolly cost a small fortune, and TradeMe is garbage these days. Could be worth checking FB Marketplace and any local e-waste facilities or the rummage section at your local landfill.

Honestly though if you think that your needs would be met by a few Orange Pis then you probably don’t really need dedicated server hardware - that stuff is expensive, uses a ton of electricity, and is extremely hot/noisy.