One thing to note: While it is worth it to practice with a cheap junk cube for learning, be careful not to start with a junk cube for something you intend to finish.
The quality of the underlying puzzle will always be there even if you make your mod nice. So when I started, for example on my Kilominx
I used a cheap Chinese Megaminx and although it looks nice it won't ever really move well.
So choose something decent, at the very least.