With 500k i would consider Land on the Thika Road along Ruiru bypass.There is this expansive land that is owned by Members of Githunguri Dairy Farmers Ranch before you reach Juja farm.
Its subdivided and every member has a title.What is usually tricky is to get the true owners who do not reside there.
Plots of 50X100 price range from 400k upwards depending on the location.Some of us have bought through an investment group of teachers.If you are interested i can refer
githukusm@yahoo.com.
The land has a lot of potential and the price of land is hiking at a very high rate and the place is yet to be built.
I would invest my 500k here rather than going to Lodong'okwe where land is almost free and not even the Government has immediate plans for the place.