With regards to the lens, be careful what you buy, as some are pretty shit. Especially the fish eye lenses (remember, proper ones cost about £600+, so your £20 one off ebay aint gonna be upto much).
Ive seen the cheap ass fish eyes in action and to be honest, there isnt much point in using one, you will get better effects by editing an image in Photoshop, and thats saying something.
Personally, I wouldnt touch any lenses other then Opteka. Ive heard of non-branded and generic makes diminishing image quality im afraid.
Remember, the more lenses you add, the lower image quality is anyway, and the less you pay, the less refined and precision made the lenses are.