It's absolutely imperative when installing a green or blue roof system, to properly consider the waterproofing layer. Typically, a seamless system is recommended, as these are less prone to failures. Single ply (epdm) unless installed in a single piece, can be prone to welding defects on the laps, as well as root penetration. A liquid applied system works well, something like the Proteus system, which is specifically designed to sit beneath a living roof build up.
My best advice would be to approach your chosen system manufacturer for design support in the early stages. They'll make all of the necessary recommendations in terms of specification for the system, they will also provide a list of locally trained and approved installers - both crucial aspects when it comes to obtaining a meaningful, single point warranty for the system.
If you're planning to undertake the work yourself, then the 'tray systems' make it perfectly viable for you to do so. As above with the waterproofing layer though, be sure to properly consider the draining, and how the water will be managed from roof down to ground level.
I love living roofs, I think they're such a fantastic investment. Absolute disaster when you need to uplift one to find a defect the size of a pin prick in some epdm though!
Kevin