Is it normal to ask to see the entire manuscript (not just the abstract) before accepting a peer-review invitation?
It is rather common in mathematics to have access to the full paper before deciding to review (it is attached or a link is provided). You could basically use what you wrote here as the basis for your reply. I specifically mean the phrase:


  I am potentially interested to review the paper, but I would like to see the content to be sure if the methods used in the paper aren't beyond my knowledge. 


I would just ask for the full paper with this reasoning. 

Only make sure to decide whether you are willing to review in a timely manner after you received the full paper.