Op the STD check and etc is a good idea but you are opening your marriage up to another huge threat....
The person who looks good on paper for your hubby and you but then will run a muck through your relationship emotionally. It could be a woman who falls for your husband and he for her and then decides to push you out. Or a woman who falls hard for you... finds out she isn't interested in your hubby and wants you. You fall for her and then hubby feeling left out throws a veto. These things seem to happen a lot in the poly world. Especially when bringing a third party into the marriage bed.