There will be no problems with warning lights
just ring euro car part ask the for a aux belt for your mk4 golf 1.8t and its a non air con model, there about £9-£13.
It will be no different from the non aircon version from factory. The slack is taken up by the tensioner anyway. Belt wrap is more or less the same. If you look where the aircon pump is and imagine it gone with a shorter belt you can quite easily see how the belt should run.