I think you are going to have to fill the holes and cracks first otherwise they will just be back as soon as any paint you put on dries and maybe look worse than it did in the first place. There just aren't any shortcuts to preparation if you want it to look right. As JRS says if the walls are not in very good condition then use a matt finish. As for the mould killer I usually use Dettox mould and mildew remover and find it gets rid of it. As with all of these products you may find that you have to use it more than once and it won't always get rid of stains the mould may have caused.