Investment Banking Contract Work

I'm job hunting in London and might be offered a contract work opportunity to do associate work on an M&A transaction. I've never worked as a contractor before, only a permanent hire. Also, I'm a U.S. citizen here on a work permit. What are some things I should consider when evaluating the opportunity and negotiating my contract?

Any additional costs I'll have to pay out of pocket as a contractor vs. employee? Exclusivity issues? Tax / accounting / income and expense reporting issues? How should I try and structure the contract - length of time, renewal features, base rate vs. success fees, hours committed, etc.? Clauses I should have to protect myself?