I want advice regarding a new maths website being prepared primarily
for the UK, but of course with an eye on the world market also.

The idea is to put online a lot of maths learning materials, in
easy-to-download form (pdf and more interactive form). Initial
subscriptions would be free or very cheap, but then the price would
increase.

I'd like your advice and answers on the following questions:
      1. What formats should we make available to download from the
website? (esp. what interactive formats)
      2. Are there any other websites that already do what we are
suggesting? i.e. who would be the main competition?
      3. What research already exists that could guide us on e.g. What
sites do maths teachers and maths students use? What are they looking
for? What needs do they have that are not currently being satisfied?

I'd be glad to pay $100 or more for a good literature review on this
area, but am also prepared to pay smaller amounts for guidance on the
other questions.