The NIHR research design service in London has a very useful website with a wealth of information and tips for how to design health research proposals for applied research (both quantitative and qualitative):  http://www.rds-london.nihr.ac.uk/How-to-design-a-study-find-funding.aspx

A particularly useful resource for qualitative applied studies is here, along with a useful presentation by Jill Russell is here: http://www.rds-london.nihr.ac.uk/How-to-design-a-study-find-funding/Qualitative-research.aspx

The web site of the International Institute for Qualitative Methodology (based at the University of Alberta) has postings for conferences on qualitative methods, podcasts, and presentations from webinars on a range of issues to do with qualitative design: https://www.ualberta.ca/international-institute-for-qualitative-methodology

On sampling, this report from the National Centre for Research Methods and ESRC, edited by Sarah Baker and Rosalind Edwards, is a useful collation of answers to the question ‘how many interviews?’ from leading qualitative researchers and PhD students – some answer pragmatically, others more theoretically. Edited by: file:///U:/Papers%20for%20QM%204%20HR/how_many_interviews.pdf