I honestly think you should just start from 2000... but then make sure you really understand every question to its full depth. At least, that's what I wish I had done rather than do as many questions as I could.
Edit: In my opinion, in April/May you want to start focusing on the more modern papers (2010-2015) as timed mocks, and then spending a week doing all (maybe not all applied in III, that can be tedious) of the questions in each paper. Thinking practically, 10 years of papers is perfectly sufficient to get the hang of STEP between now and then.