hi tasneem, welcome to the forum. my thoughts on your 4 choices:
1. Canon 60D
don't let the plastic thing scare you. my wife has a dinky little Rebel XTi that she flogged rather mercilessly ... still in working order today. unless you are willfully abusing your camera (and I rarely see anyone do that), there's no reason to get stressed over the build quality of the 60D. it's an excellent camera with some great capabilities that I've had no qualms about recommending to my friends, and they've been very happy with it.
2. Nikon D7000
excellent camera, I can't speak for the price. only you know how much you can really afford to spend. to me, in all honesty, if you are a beginner, get a cheap camera and learn with it and abuse it. one of two things will happen with ANY of these bodies: 1) you will outgrow it and upgrade as your skills improve, or 2) you will find that it is just right for you and happily use it for a long time. they are all fantastic choices, so ... pick what works. save the money for a better lens, or for multiple lenses, that's what will bring you real joy in photography and allow you to shoot differently than everyone else out there with a kit lens.
3. Nikon D5100
rumors are rumors. from what I've seen, this did not drop today. everyone on the forum knows how I feel about waiting to buy gear ... buy what works for you and I doubt you'll regret it.
4. Canon 600D
also an excellent camera, for a great price. choosing between this and the 60D can be hard since there are so many similarities. I'd say go to a store, and see which one feels better. I would pick the 60D for two very simple reasons: viewfinder size, and AF module. both of these are vastly better in the 60D and worth the slight uptick in price. but again, you pick your price range.
don't discount going online and shopping for older cameras. the Canon 50D is still an awesome camera at a great price. the 550D is equally a very great camera for the price. In fact, you could go a generation older (40D or the XSi) and still get a fantastic camera. despite owning and mostly shooting with my 5D Mark II, I have no problems with picking up my old 30D and taking some great photos, nor do I have any hesitation to shoot with my wife's current T2i (550D). learn the art of the photography, not just the specs of the camera.
good luck and have fun shooting.