Thanks for the welcome.
Budget and location:
We are located in Canada, and have a budget of about 1500$, which he would be very hesitant to exceed.
Aims:
The computer will be used strictly for my dad's photo business, and the only intensive application will be editing photos in Photoshop CS. The resolution of these photos can be rather high.
Monitors:
He will be running one monitor. We are not certain on the resolution yet, as we are planning to upgrade this as well, but are considering 1440p.
Peripherals:
We do not require a new keyboard or mouse, but as stated, we are looking in to a new monitor.
Reason for upgrading:
He is upgrading because his current computer is quite old, has some performance and stability issues, and he wants to alter his storage and backup system.