I would have a good close look at the joinery and dimensions of the main frame first. The panels will prevent the frame collapsing sideways. If the construction looks solid enough, and as I suspect the drawer frame is of 50 x 50mm timbers, I would go for it.
Maybe as an extra precaution, place a sheet of 18mm ply over the top for added strength, and a sheet of polystyrene to sit the tank on.
Oh, and remove the little feet from the bottom too so the frame sits nice and flat on the floor.
My reasoning for all this advise, Im sitting at a similar desk as I type and would not hesitate to place a bigger tank on it.
I have a 450 x 450 x 900 tank on a Criterion buffet unit that is not as well constructed with no problems.
You will have great storage in the desk for all your fishes bits and under it for the filter.