As I read, you are considering getting a new PC plus somethings to be a backup destination for your photos.
Speaking on an external hard drive, I personally would be very cautious with trusting a single disk with my important stuff. I've been an IT guy since early 90's and I've seen and lived through plenty failures, including memories gone due to a failure of a Lacie disk. I personally use NAS devices that host multiple redundant disks and synchronise via the network, but, I'm afraid, such a setup might be too much for someone less technical.
I just Google'd on "Usb 3 Raid Enclosure" and my favourite IT store in Sydney showed 15 options, albeit none of the brands listed there are familiar to me. And since those enclosures are designed to host 2, 3 or 5 disks, the portability will be an issue, you if ever had a need to move that thing in your handbag.
The below one looks quite fast. This might be over the top, but a five disk setup would be a great from both redundancy and performance aspects.
TerraMaster D5-300 5-Bay USB 3.0 Type-C Raid Storage System
Speaking of computers - I do custom builds since that allows me to get specifically what I want while saving money. Windows 10 is such a wonderful OS, stable and fast.
Subject to you specific needs I would recommend for a decent/trusted vendor (again, I assume you are not willing to be too technical) my recommendation would be:
1. SSD that will host you Windows and applications as well as the Photoshop scratch disks. 512 will be a minimum, 1TB is better
1.1. If this workstation comes with a raid setup -- say SSD is the system disk and then it has three magnetic disks configured as a RAID, that would eliminate the need to an enclosure I descried earlier
3. You want the latest generation CPU which is Generation 10 for Intel. Whether Core 5 or 7, nothing crazy about the gaming editions or overclocking. Core 7 would be faster, but more expensive. Generally, you'd want to get as many cores as your budget will sustain. I cannot recommend AMD-based systems -- since the x486 days they always have dangled a promise of something better whilst cheaper... But in my experience, they never delivered. About 5 years ago I went firmly with Intel,
4. RAM - minimum 16GB but 32 is better. On my PCs, both of which have 64 GB, Phtotoshop comfortably chews 12-20GB on top of the Windows and other apps itself.
5. I cannot recommend all in one PCs since they will unlikely offer a good quality monitor that is important in photography editing. I might be wrong here but glossy surface works on me like garlic with vampires. I'm a believer that a monitor should not be a mirror and speaking of photo editing, its most important characteristic is "as close to 100% Adobe RGB" as possible.
6. Fast GPU with plenty of RAM. I use nVidia GTX1080 with 11GB -- works very seamlessly.
ps. the recommendation is based on my own, very subjective preferences and quite extensive experience in this area. It might be over the top and more expensive compared to the mainstream options you'd see in a shopping mall.