OK In Qld this first option should work really really well.

Water the ground really well and even give it a little bit of liquid fertilizer. It should make the weeds very very happy. Then once they look nice and green (probably a week or so) put some black plastic (a few garbage bags will work just as well) and hold it down with some bricks or weights or pieces of wood. After a couple of weeks (or even less) they will all be dead. They effectively cook under the plastic if it has been warm.

Alternatively, if you are really keen to get going mix a up of salt in a litre of vinegar (it will take a bit of stirring to get it all mixed in) and then on a warm sunny day spray or water it onto the weeds inside your new box. It will kill them within hours. But a couple of days later give the whole area a good water to dilute the vinegar.

Also, before you start building the bed on top put quite a bit of gypsum at at the bottom of the box as this will over time make the soil underneath easier to break up and help with water penetration. Then cover it with a layer of cardboard or newspaper and wet it down really really well before you fill your box.

As for the type of soil you put in - it depends on how big the box is. You need to work out how much you need.

Here is how to calculate it - length x width x height.
So, if it was 1m long by 1m wide by 20cm high you would need 1 x 1 x 0.2 = 0.2
This means 0.2 cubic meters or 200 litres.

So either you ring a soil supply shop and ask them to deliver you some - but usually they won't deal with less than about 2-5 cubic meters - or if you have a trailer you can usually go to their depo and get it (a trailer generally holds between 1/2 and 1 cubic meter) or if you are talking only 200ish litres the easiest thing would be to go to bunning and buy it in bags.

As for what you want - the more organic matter the better - so ask if they have cow poo or similar and get half half soil and cow poo - or if you are buying it in bags get a few soil, a few cow poo, a few mushroom compost and mix it all together.

And then - put a big layer of mulch like pea straw or lucerne or something over the top of it all to help retain the moisture.

Then go wild and have fun