A computer tells you, among other things, your ascent rate, which is both vital and difficult for a beginner to judge. There are endless other items that some diving gurus insist that you need and, depending on where you are diving, some may well be essential. However, the only other truly important item is a diving suit. You put on the bits you need according to the temperature of the open water in which you find yourself diving. It will set you back around 170.
Diving equipment needs little in the way of servicing.
It is more a matter of looking after it by rinsing it in fresh water and storing it somewhere dry and well ventilated. Only regulators need to be serviced annually, and that costs between 20 and 50, depending on the type. You might have noticed that there has been no mention of buying a diving air-cylinder. So you may not need to buy a cylinder of your own. The cost of an individual dive depends very much on where in the world the diving centre is and whether you buy a package of dives.