Although I can see that you are already well on your way with your wonderful idea Danyel72 - as I know that you want your memorial garden to be extremely special and meaningful - I have some further thoughts too - especially if you are thinking of a fountain as focal point - like for instance - you could compliment the fountain with plant varieties that
contain your brother's name or the name of something that he had a passion for
or ones that have a particular scent that he liked or a scent that reminds you of him
also any favorite flowers or trees that he may have had or any plants that remind you of a particular time or occasion
or you could choose plants to reflect your brothers personality - like for instance if he was lively and full of fun - you could surround the area around your focal point - with warm lively colors or gentler paler shades if he was quiet and reserved.
Either way what ever theme you choose to go with - I feel more than sure that it will be a beautiful and especially love your idea of adding lighting