Do you mean infection from a second hand tank? If so, a thorough clean will do the trick.
Fish do not need light but plants do. If it is in a well lit room, and you choose plants which don't require a lot of light, then having no light won't be a problem.
If you want a cheap light, get one of those compact fluorescent tubes that look like folded over rods around 10 - 15cm long and that plug in like an ordinary light bulb. These do not get as hot as light bulbs and will sit on top of a sheet of glass (like an aquarium glass lid). You then go see a friendly builder, or visit a building site (or shop) and acquire a piece of plastic guttering the length of your tank. You can get end caps for the guttering if you want and cut a shape out of one end for the power cord to fit and there you have it. Sit the light on the lid and put the guttering on top and it looks very neat and tidy. If you like, line the inside of the guttering with aluminium foil to reflect more light downwards.