Good afternoon Ally, and welcome to the mot helpful and uninintentionally complex forum in the whle world
These guys are clever... but they DO have their own thoughts on everything, so the selection will be hard as nails, as there will be no RIGHT answer,,,just a series of close to perfect ones!
The critical part in EVERY PC build is QUALITY items.
Don't buy a cheap PSU, because when a PSU fails, it takes many items along with it.
We'd all suggest good ram, something branded by the likes of Corsair, over and above stuff with generic/ no label stuff.
Hard drives should be from Western Digital, because almost everyone here agrees they're the best, or not shy of it. Some people prefer Samsung, but trust Western Digital, some people love Hitachi (me) but still totally trust Western Digital. Some ove seagate, but lots don't and they all like Western Digital. Get the picture?
If you KEEP that monitor that you've got, you'll only need the most basic of graphics card sin todays market to fill it with lovely games pixels... but we'd all sugest , I think, a bigger newer one.
But do decide on the amount of desk space you want.... they're shalllow but WIDE!