I have 2
Inkscape: http://inkscape.org/
GIMP: http://www.gimp.org/

Inkscape is an AMAZING vector software and GIMP is just as powerful as Photoshop
whichever one ya choose I can show you how to do almost any cool thing you want in it

something I did in GIMP recently: http://www.furaffinity.net/view/5455260
something I did in Inkscape a while back: http://www.furaffinity.net/view/4691375/

GIMP. Totally seconded. It has the ability to do more subtle color and shading work, too (although when it comes to digital work, I'm too inept to show a good example of such)... And it *is* free.

Inkscape is also a great choice it very easy to do cell shading and Anime style work.

From what you posted, I can totally see it, but with no experience with it myself, I can't say either way.

if you don't have a tablet i suggest using Inkscape since you can easily reshape lines nicely without having to erase.