The syllabus for STEP I and II is the same and consists of C1-C4 + M1 + M2 + S1 + S2. STEP III consists of C1-C4 + FP1-FP3 (+FP4 depending on your board) and it's unclear what applied modules are needed for it. You can find a detailed explanation of the syllabus on the website for STEP.
If you're doing it for the Warwick offer, I think you'll find getting a merit in the AEA a bit easier.
As far as tackling STEP goes, I'll let others answer. You might be better off asking in the maths forum by the way.
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