My mid 2015 iMac t The Power supply unit is failing suffers from random power off, remove power plug rest for several hours before a restart is viable. Is there any relief or advice on the replacement Power board.

I had an iMac with the same problem. It could be rebooted as soon as the power cord was removed from the back and then reconnected. The machine was under warranty so we were able to get it repaired by Apple at no charge.

You can get your iMac repaired by Apple or an Authorized Apple Service Provider. Unless it's still under warranty you'll have to pay for it. It's a repair that's not for the faint of heart.

These are just user to user forums and most users don't know anything about repairing hardware so you need to be patient for someone to stumble across your post that may be able to assist you.

While a bad power supply is a possibility, the problem could also be with another component as well since you are effectively performing an SMC reset on the Logic Board and draining power from all the other electronics too.

The only power supply you can use is an official Apple power supply made for your exact model, but Apple doesn't sell parts to consumers. You may be able to find refurbished or used parts online, but you do need to be careful.

You don't mention your technical hardware abilities. People should not be opening an iMac at all because the power supplies on the later models such as a 2015 model is completely exposed which can cause a bad shock or even death if you don't know what you are doing. These power supplies can retain a charge for a long time after the computer has been unplugged from the electrical outlet. Only an experienced tech should open up an iMac especially when replacing a power supply. These newer iMacs are not easy to work on and contain very fragile & delicate cables & connectors that can be extremely difficult to disconnect & reconnect as there isn't much room to work or even see what you are doing. It is very easy to damage these cables & connectors.

Unless the fan is making noise or not operating correctly there should be no need to replace any of the fans (the iMacs have multiple fans).

Take the computer to your local Apple Authorized Service Provider to be professionally diagnosed and they can quote you repair costs. If you bought the computer refurbished and also bought AppleCare and you have any AppleCare coverage left then Apple would pay for it. Other than that, you have a 5 year old computer that needs repair, you cannot expect Apple to pay for it or do you?