This hot season, you can start with the garden base... soil additions, placement, finalization of elevations, etc.. including of course the drainage and electrical lay outs.
Tapos lagyan mo na ng grass. Like golf course grass, it will thrive in this hot weather... Basta you have it drenched dilig before 830AM; and then again in the afternoon... pagnaka singaw na the heat. Do not water in the afternoon while mainit pa or the grass feels hot to the touch.
Just those works will give you a lot of return per peso investment. It is gonna be a big deal for you all to have that wide green expanse. Saka mo na utay utayin the rest of the plants.
I forgot... have metal works fabricated to encase the water tank... you can ask your landscape architect what to do about it. Or, kaya mo yan... just think it out including the practical considerations required for cleaning and maintenance. I have seen some tanks encased in trellis like metal works tapos pinagapangan nang creeping plants.
Work on it soonest coz well, the experts say that in terms of per peso investment, it is the garden that gives the best bang per buck.
Yours has the added advantage of space/size. Open space is the new luxury feature nowadays, remember that.