It´s impossible to do a recommendation like this. I read your posts and i can´t tell what type of software you will be using.
 
Keep in mind that Sample based instruments like Kontakt wont ask for much CPU, Emulation of synthesizer stuff like Reaktor will, why? Sample based does not involve many calculation, unless you add mastering/fx vst on top of it. 
On the other hand software like Reaktor is pure mathematical, everything you do is cpu processing.
Same for FX/Mastering VSTs those will use a lot of CPU.
 
So if while you write your songs you use many CPU heavy synths like Reaktor/Massive/Absynth/Arturia stuff/etc you will want a good CPU or you will have to keep freezing the tracks.
 
Audio tracks (recording or playback) won´t use much CPU, unless (again) you add mastering and fx stuff on top.
I have seen i3 CPU fail with simple 48K 24b audio track + guitar rig and izotope ozone on top of it.
 
For me, anything bellow an i7 is a no go, but that´s because the type of projects i do, there´s too many vsts and too many fx stuff. You will have to understand what´s the minimal setup for you, don´t you have friends who might let you try their systems?