-
Any weight that the plasterboard fixings can handle, the point where it's fixed to the plasterboard would be the weakest link.
Maybe you are not getting many answers because there are so many ways to do it and you are going to have to make compromises based on the material you have to hand. If we had you inventory listed then between us we could come up with the best solution but there are so many possibilities.
If you have decent size pieces of the Mdf then you can spread the fixing point and you can also screw batons onto the Mdf from the wall side through to the batten which would give you a wider support. If you have enough you could use strips of Mdf resting on the floor and lightly screwed to the walls since supporting the weight of the shelf + booze is your main concern.
Still trying to work out the best way of making a counter top for above the out door freezer.
I've got ~75x75mm posts which although nice and beefy, seem overkill. Also because I'm limited on other appropriate wood they'd need to be screwed in to the wall. Even counter sinking them halfway would require quite long screws which I need to order.
I've got some 20mm MDF. If that was screwed to the wall, what sort of weight could you put on top of a counter placed on that?