I suppose that they are just there to prove that you've thought about it and haven't just decided on a whim.
So, most likely all of your answers will be on this thread 
The reasons why - I'd talk about what has brought you to the point you are at now, whether it's been a slow burning thing or a sudden development triggered by a specific event.
What problems - Again these should be easy enough to find by scrolling back through the thread, keeping a diary for a few days, or look at a list of ASD symptoms and pick a few which are relevant to you and provide examples.
Why will it make a difference? - That is fairly simple, I think. Either it's for extra concessions at work, to be able to access appropriate treatment (for something like ADHD which can be medicated, or in terms of therapy, seeking out self help strategies and literature, etc) or just for your own peace of mind and/or self image.
Pros and Cons: The first which comes to mind is better legal protection, better able to find appropriate resources to help self, but risk of becoming "labelled", risk of "othering", risk of depression when diagnosis does not solve all problems.
What assessments - I should think it would be okay to say none, or list any you have had including assessments for depression.
What support - This one is difficult, I suppose you could say that you're not entirely sure and you would like to ask them what the assessment process involves as you're not sure what sort of support would be appropriate. I think I'd like to be assured that I'd have an appointment with somebody to be able to talk through the results afterwards and discuss options (including the option to do nothing) rather than just be given a yes or no kind of result.