I get the convenience factor of buying a pre-built desktop, but it is REALLY, REALLY, REALLY easy and far better to build your own desktop.
CPU,RAM,SDD,HDD,GPU, MOBO, Case, Monitor, Keyboard, Mouse, OS license and heatsink for CPU.
That's it.... practically plug and play. It was something my father taught me when I was sub 10 and I have been able to build amazing PCs for cheaper than a box store.