The 12 megapixels cellphone camera will give you a large, high-megapixel, blurry, noisy and unsharp picture (one exception, high quality and expensive smartphones (like the iPhone 4S you mentioned and others in the same price range) in good light are actually quite good).
The cheap fisheye-for-cellphones is cheap because it's a toy, it's not made out of optical glass and it's not designed to fit the camera optics exactly - it will degrade image quality even more - the result will be a very low quality image.
The truth is you get what you pay for, 100 euros is not enough for acceptable optics.
Here's your option for a fisheye camera (from cheap to expensive and from worse to best):
cellphone with a toy fisheye adapter - as I said this will give extremely poor results, you can consider this if this is the only option that fits in the budget but know what you will get (a crappy camera with an even crappier lens will never produce good pictures).
try to get a security camera with a fisheye-ish field of view - this won't produce high quality images, and probably won't have the full field of view you are looking for but it will be better than the cheap cellphone with the plastic toy "lens" in front of it.
A point and shoot from a real camera company (Canon, Nikon, Fujifilm, Sony, Pentax, etc.) with a fisheye adapter designed by the camera manufacturer for that specific camera - if you can find one of those for an acceptable price this will give acceptable results.
A mirror-less interchangeable lens camera with a good fisheye lens - I think this is the best option for you, it's out of your budget but this can actually produce really nice high quality results.
A DSLR with a fisheye lens - image quality will be only a little bit better than option 4 above and so far out of your price range it's not even worth mentioning.