Background: I am a first-year PhD student in pure mathematics studying subject X. With my supervisors, I recently published on arXiv a paper on a particular example A that explores how X works when applied to A. My contribution to this paper was partial (a couple of proofs and calculations) but I did read and check the whole paper before submission.
I have recently been asked to referee a paper for a reputable journal also on the application of subject X but to a different example B. The two examples are distinct but are of roughly the same flavour overall.
Initially I thought that I was too junior to agree to referee this paper. I am only a first year PhD after all and my only paper so far hasn't been peer-reviewed yet. But having looked through the paper, I do believe that I am qualified to judge its veracity.
My main concern however is that I do not feel able to judge whether this paper is substantial enough or the results are new enough to merit publication in this journal. My opinion having read the introduction is that the paper is an interesting application of X and is worthy for publication. However, I have not read many papers from the journal, and the total number of papers I have read is small so I do not necessarily feel qualified to judge whether this paper is suitable for publication in this particular journal.
Despite this, I still think I could provide a useful review in the sense that I could do one half of the referees job: that of verifying the results in the paper. So my question is this:
Question: Should I agree to referee this paper even if I do not feel I can judge whether the paper is worthy of publication on the grounds of importance and interest? Should I talk to the editor about this before accepting?