You might like looking that the Puget Systems website. They build custom computers, and their website has a lot of excellent information on recommended components and discussion of why those were chosen. Of course, you don't have to go with exactly what they are using, but it makes a good guideline.
https://www.pugetsystems.com/I did some upgrades to my desktop computer last year and Puget's website was a big help.
The SSD should be no less than 500GB, and this is where you will store your operating system and all other programs that you use on your system. Your internal HDD should be at least 4TB so that you are not likely to run out of space any time soon. This is where all your documents and photos will be stored.
The HDDs need to be good quality, such as the HGST Ultrastar, which has a 5 year warranty. I also use another 4TB HGST Ultrastar as an external drive for backing up photos and documents, installed in a case with a fan. It is easy to do, just insert the drive and plug it in! More than one backup drive is recommended - I have another 4TB WD My Book for backups. I use a Dell Monitor 27" and I like the size very well. I also have space limitations! The one I use is an UltraSharp IPS monitor.
Additional things to consider: Adequate size power supply, dedicated graphics/video card, motherboard.
The nice thing about a desktop is you can have more than just 2 internal drives if you want. The most important thing is compatibility of components. When I did the upgrades, I was limited to choosing components that were compatible with my motherboard, which I was not replacing. Eventually it will get replaced as well, because it is older, and then I may also end up having to also replace the RAM, depending on the motherboard I will install.
You might like looking that the Puget Systems webs... (