Nirvana is a good choice, but most of their songs are tuned down a half step as I recall. Green Day might be an idea - 'When I come Around' and 'American Idiot' are very easy.

Depends what you want. I think you can learn anything if you put the time and effort in. I took on 'Eugene's Trick Bag' a long time before I had the skill to play it - I just really wanted to learn it - and I did. I could play it perfect, but a bit slow. It's now some years later and I've developed the motor skills to do it at full speed...

Anyway - what song do you really wnt to be able to play, regardless of difficulty? It's good to have a challenge, and maybe to run that in tandem with something easy, so you get results quickly from say a Green Day tune, but then the satisfaction of a job well done by doing something more difficult...