If you're getting paid then you will need a back up body.
Cards are only a problem if you remove them before the read/write action has finished- so either use a very big card or lots of small ones, and wait until the red light stops flashing.
Used bodies are nearly always good- I've never had problems with used kit. No guarantees with rental kit once you start using them. You cannot tell when something will go wrong- hence backup body.
If you are going into the business of photographing a wedding you had better know your stuff because there are no re-runs and it is not fair if you produce mediocre results.
Otherwise go for it.