If you are concerned about how much of a spanking etc. a person can take, it is probably best to have a "safe" word to use during your session. By safe word I mean a word that would be used out of context during play, such as red or umbrella. That way if you are hurting the client too much he can use the safe word, and you'll know to stop or ease up. Don't use the word "stop" though, as experience has taught me that submissives like to beg/plead with you to stop whilst playing. This is part of their role of been a submissive.
As for a contract, Jessi is right about not been able to give their consent to assualt. It wouldn't stand up in court. Check out the link below
http://www.dailymail.co.uk/news/worldnews/article-1289216/Racing-boss-Robin-Mortimer-dies-sadistic-sex-session-perverted-dominatrix.html. This is an extreame and rare case and Robin was a heavy user of poppers which in the end caused him to have a heart attack (although it is not mentioned in the article), so don't be put off. It's just to give you an idea of how safe you want to play it.
You'd also be hard pushed to get anyone to sign one, as most clients are paranoid about their privacy, and would fear that the contract would be used other than in the session itself, or fall into the wrong hands. It is however a good idea to ask if a client does have any health problems etc. before you start. That way you can gage how far you want to go for your own safety as well as the clients.