we want to replace our desktop with our new gen 8 iPad is this wise?

Nice idea. Wrong choice of iPad.

You're trying to replace a desktop computer with a $350 appliance that doesn't even have a modern display.

Look at the iPad Pro and Magic Keyboard and some USB-C and Bluetooth peripherals.

Now the price and value are in the same ballpark as a cheap laptop from Apple.

A couple of ideas, if you have a Costco nearby take a look at the nice 1TB SSD drive they currently have in stock.

If no Costco near by take a look at the offerings from OWC they have lots of options and the support and service from them is "Top-Notch" - https://www.owcdigital.com/products/external-storage

It all depends on a bit more detail on how you use your Desktop.

The 8th Gen iPod is a very good machine and using iCloud to store and manage your photos and music can be done on either the old iMac and or the 8th Gen iPad or on both. The iPad is portable so that is a plus for it.

I would suggest that you keep your old iMac up and running and make sure it is all updating to iCloud. Purchase an 8th Gen iPad and try it out. In the US you can always return it within 14 days if it is not what you would like.