Where to start growing on a plot of land...

[Penellype]

Grass turves can be used to create loam by stacking them in a heap and they rot down - I have done this myself when I created some beds in a garden that was almost all lawn and it does work, although you do get some grass growing back. I suspect if you turn the grass over some of it will rot and some will regrow, but it does depend on the type of grass (there are lots of different types). Annual grasses such as rye and meadow grass will rot down, perennials such as creeping fescue and couch will regrow from bits of root. If you turn a few bits you will see what the roots look like - anything that is long, straight and white is couch (or similar) and needs removing. Even so, turning or covering the grass will kill off the top growth and any annual plants and you will be able to make a better judgement of what is left.

I would agree with the suggestion of raised beds, even if you make them without sides (for an area your size making raised beds out of any edging material is going to be expensive). My allotment is very wet at the bottom end and my plants would have been under water for months if I had not used raised beds. If you have a very wet area it might be an idea to dig a wildlife pond, which will help attract beneficial creatures to your plot, and you can use the soil from the pond area to raise the level a bit elsewhere.

If the soil is poor and waterlogged, add as much compost and organic matter such as leaves as you can find. It may also help to add some sand to your beds to help open up the soil structure a bit, but I would concentrate on getting rid of the grass and planning your layout first.

[ameno]

I've found even couch will die as long as you bury it deep enough. It likes to be near the surface.
So if he has couch, I would recommend digging out turves and stacking them upside down as tall and narrow as possible, then covering with black plastic or tarpaulin (basically anything completely opaque and weather resistant). After a year or so the couch should all be dead, and the soil can be dug out and used again.