I would agree, building will get your more PC for your money. What I did was look around for recommendations for each part. So I built a fairly budget-mid range machine and looked for reviews of CPUs, motherboards and GPUs first. Everything is designed to slot together and it's fairly hard to completely screw up.
Make sure your motherboard is supported by risers - it doesn't screw directly into the case.
Some cases have 'cable management'. What this means is they are designed so there's a decent gap behind the mobo and there are holes cut through the structure of the case which allow you to thread you cables through tidily
SSDs make the biggest difference
Installing a CPU is not nearly as scary as I thought
It's quite hard to mess up, the connections for your cables are designed to only let things go in one way
The whole thing is a kit. So you buy an optical drive, it needs to be connected to the mobo and to your power supply
Same with the hard drives. All the cables allow you Graphic card and memory slot in to set places. The mobo connects to your power supply in 2 places. Many cases come with fans which you just plugin.
If you can replace memory sticks or switch out the graphics card, you can do it.
I really recommend
https://uk.pcpartpicker.com/ (there's a US version too). You can get some ideas from there.