Update:
Seems the tank will be built on site by Greg. However, 15mm would double the price. Thats why the plans changed a bit and the tank will now be 12mm glass 250x80x65cm, 60cm water = 1200L. Also kids decided that it will be South America (planted) - tiny fish in a large tank.
The overflow to sump will be placed on one side - probably following the design by beananimal to keep it silent. Sump will be app. 250 liters about 60% filled when running, sponge for mechanical and ceramic for biological filtration.
Next step will be a plan"e for the steel stand.