I'm sure you can find a pure-breed sheltie pup out there... You should difinitly consider the risk you are taking... I mean the bictes can get diseases and spread them through the pups...
You and your wife should sit down and think about it and take into consideration all the risks.. Also you have to handle the bitch's seasons... And one of you has to be home all the time... The most important fact is that you will be only adding to the never-ending surplus of cats and dogs... I'm only asking you to think... think about the dogs all around the world, too many dieing each day because there are WAY too many... First let's give home to the ones who need them and then we can start have more...