How to decide for the contamination value (proportion of the outliers) in my dataset?
If you want to experiment something like that I think you can add some outliers into your data for that respective field and see what are its effects.

If you find that these are prone to outliers(most of of the features are), then you can handle situation by removing such outliers before appending to the existing data.

What is your target variable?
Did you do any kind of Correlation analysis/ Predictor Importance?