I wish to install Adobe Photoshop and was wondering if it is possible to purchase the latest version as a one off rather than paying a monthly subscription ... ?
No, Photoshop CC is subscription-only. You can opt for an annual sub rather than a monthly sub if you buy a pre-paid card at a participating retailer. (At least in the USA; not sure about the rest of the world.)
Maybe Photoshop Elements is an option. It's code is built with the Photoshop engine, but lacks some features for the professional production. It comes with a permanent license.