As somebody who knows nothing about Forensics or lab based science and not all that much about writing a proposal, but who is starting on a project very much within the social sciences...
READ.
Until you have some idea as to what you are interested in I suggest reading books and journals. Writing a proposal that 'ticks the boxes' is probably going to be the easy part. If you do not have a question that you think is interesting and useful but you cannot find an answer to then you are probably not going to be that excited about embarking on a PhD.

Have you already got experience doing non-lab based research? Just because you aren't using expensive chemicals does not mean you do not need to have a solid background in methodology. Are you going to do a grad certificate first to cover the relevant fields?

Also, I know you are being honest, but if you say you are going to look at a non-lab based question as a 'cheaper option' then you are probably not going to make many friends.