Slope of Plot or Site plays important role while considering to purchase a plot or house. Vaastu tips contains some guidelines to help you in choosing the Plot.
The Northeast half of the plot is called the Solar [Sun] half and the Southwest half is called the Lunar [Moon] half. Preferably the Solar half [Sun] should be lower than the Lunar half [moon]
In short, Vaastu recommends:
Northeast corner should be the lowest
Southwest should be higher than the Northeast corner
As in broad the plot should be elevated in South and West directions, and declining towards North and East, so that water should flow from West to East and from South to North. Such plot is known as elephant’s back. Goddess of wealth, Lakshmi is said to live in such Plots
This type of plot brings Joy, Happiness, Good offspring and overall success in life of the residents in such houses.
On the other hand, half part of Sun if elevated over Moon’s part, the family residing in that house will turn into poorer day by day, health problems and get involved in unwanted quarrels, litigation and lose everything gradually.
The plot, which is raised in the middle and low all around, is known as Turtle’s back. Residents in such land leads to greater enthusiasm, luxuries and accumulation of wealth. 
Though, any of the deficits observed in your plot [After purchasing] please make the plot Vaastu friendly like elevating Southwest than Northeast before construction of a house. A plot with raised West and slope towards East will give substantial growth and prosperity.
Plot signifies an immovable form which radiates positive or negative energies depending up-on its site, alignment, outline, slope and quality of soil.
While choosing a plot five main points are to be considered. Generally, Vaastu consultant services are not used while purchasing a plot. My suggestion is, it is good to use services of Vaastu expert while selecting a plot, as this could help in deciding the proper Site.