The following papers may further help, particularly the guidance/advice as to the review of literature in research proposal, for example: Sidik, 2005, pp. 30-31; Faryadi, 2012, p. 113; Klopper, 2008, pp. 64-65; Kilbourn, 2006, pp. 553-558, etc.
Abdulai, R. T. and Owusu-Ansah, A. (2014) Essential Ingredients of a good research proposal for undergraduate and postgraduate students in the Social Sciences, Sage Open, 4, 3, pp. 1-15.
Faryadi, Q. (2012) How to write your PhD proposal: A step-by-step guide, American International Journal of Contemporary Research, 2, 4, pp. 111-115.
Kilbourn, B. (2006) The Qualitative Dissertation Doctoral Proposal, Teachers College Record, 108, 4, pp. 529-576.
Klopper, H. C. (2008) The qualitative research proposal, Curationis, 31, 4, pp. 62-72.
Sidik, S. M. (2005) How to write a research proposal?, The Family Physician, 15, 3, pp. 30-32.
Literary review should be contended & nor more than ten years. if your work is original and new area of research then you ll not find much review then you can extend the years to fifteen
My supervision always mentions, that you never have enough literature. In case of your proposal it just a beginning. Thinking back to my proposal I already had almost 500 different articles in my Endnote, finally I used roughly 20% of the sources for my research proposal. The most important thing on you research proposal is to use the literature that supports your research question and research approach.