The monitor is definitely disappointing. 5ms response time, 1080p, I'd definitely replace it immediately.
The G15's lack of mechanical keys, and outdated digital display cries for an upgrade. The DeathAdder could be upgraded or replaced entirely by a far better company like Roccat or Logitech. But if the mouse still works, and perfectly satisified with how it feels/performs, then I wouldn't worry about it too much.
I would personally prefer having a liquid-cooled build, but if you have good enough air flow and don't mind the noise, getting another CPU cooler (preferably from Noctua o