Raptor, your idea of fixing a batten below the unit does give it a lot more strength but I would still not be happy using plasterboard fixings.
No plasterboard fixing will hold a fully laden 1000mm wall unit. You need to fix a batten (you will need to put one at the bottom as well to keep the units level) along the wall securely fixed to the studs and screw into that.
Other option is to insert a decent sized blocks into the cavity of the wall to spread the load but this is more messy.
It is also possible to put a batten just above the unit and screw into that as it will be hidden by cornice but it is a lot more tricky to get a strong fixing this way, you would need to use brackets.
It does depend on the layout of the room.
I went to a house where the wall units had come down, what a mess. This owner made jam and had stored dozens of bags of sugar, together with enough jam to stock Sainsburys, and put it all in the wall units. They shovelled the mess out in wheelbarrows.
PS I didn't fit them, honest