Yes, you need to get a contract. No, that email isn't going to cut it.
Artstation.com have a good article on artists contracts and why you need one. Plus some advice on how to modify and negotiate. They also link to Artpact.com who provide some free examples of pre written contracts
The link --> http://magazine.artstation.com/2015/09/artists-guide-contracts/
Well worth the read.
cheers
Sean