As with any electronic device there are many factors to consider and karaoke machines are no different. The main thing to consider is how you are going to use the karaoke machine. If it is a toy to play with then it would not make sense to buy the top of the line model with every feature. The other side of that would be that you would not want to buy the cheapest model if you were planning to use it in a business establishment. The basics of karaoke systems are the same, there is a cassette or cd player that plays karaoke cds (which are cds without the main vocal track) and a microphone that allows a singer to replace the missing vocals with their own voice. The lower end (and least expensive) models do not have much aside from this. If you do not require any features then this is the option for you. Often these machines can be found priced between £35 and £50.
A feature that many people like is the ability to hook up the karaoke system to a monitor that will display the lyrics along with a marker that informs the vocalist of when the word is supposed to be sung. Sometimes this monitor is part of the machine and sometimes you can hook the machine up to a television set or a computer monitor. Often this level of machine will have a few features such as adjustments for the ratio of vocals and music. Then there are effects that can be added to the voice, the most common of these would be reverb. Reverb is an effect that mimics the way that a voice would bounce (or reverberate) around an environment such as an auditorium. Because most professionally recorded vocal tracks have reverb added to them it sounds more "natural" to add the effect to the singer's voice.
The final primary feature of a karaoke system would be the ability to record the performances. The quality ranges more in this feature than many of the others. To make a recording for you and your friends to have fun with you do not need a high end machine but if you are wanting to provide a professional level recording the karaoke machine will need to be towards the top of the line. These machines can run anywhere from £150 to several thousands of pounds.