It depends on the kind of weeds. Covering it with cardboard could help.

I don't think it's that they let it go (on purpose) as much as it was that they were busy with their move. If the garden condition wasn't mentioned in the contract, we could hardly blame the previous owner for leaving a garden area unattended. Based on the pictures, it just looks like a fenced off section of grassy area, but maybe that is what you mean by letting it go.

However, regardless of the reason, for such a large space, I would make sure there aren't any pipes that can be affected, then rent a tiller and go through the area.

Either that or, using hand tools, work up small sections at a time, such as a 4x4 area. As you mentioned that you might want raised beds, you know that you don't have to till up or weed up the entire 45x40 area.

I don't know if they let it go on purpose but they were tending to it when we saw the house. I had a garden at my previous home that I continued to tend to until the home was sold as I wanted to keep the house in the original condition. But I sometimes forget that not every seller is as diligent as I am. House was pretty well cleared out the month prior to closing when we came for inspections. We were also left with a yard infested with fleas. Just a bit disappointed at how some things were handled. But it will all work out. Will most likely do raised beds.

Covering the entire area with large black plastic sheeting typically found at big box stores, will deprive the entire area of sun, water, and air. Keep in place for as long as possible... even throughout the winter. The weeds will die back all the way to their roots. Seeds in the ground will also be distressed and will die. Then you can dig or till the area and add in compost to allow for fall or spring planting.