Should I read  ingredient weight as prepared or unprepared weight?
In your example, it means 500g pumpkin before preparing. 

In general, you might see this two ways:


500g pumpkin. Dice the pumpkin.
500g diced pumpkin.


The first option refers to the weight before prepping while the second refers to the weight after prepping. In general, the second one is far more exact for the actual recipe while the first one is more exact for shopping. I assume ease of shopping plus presumption of yield is why the first one is so often used, although in high end cook books (Grant Achatz, Thomas Kellar, etc) the weights are often given post prep for precision. 

You will see the same thing for volume measurements (i.e. one cup of nuts chopped vs one cup of chopped nuts).