The Duel Adapter is $120US, that is all you need with a MacBook Pro.
If you capture via the HD SDI there is not connection to the footage on the P2 cards, so if something happens within the edit and you need to access this those files it will not be just a reconnect but it won’t be the end of the world either. But you are entering into a mixed format world, capturing some footage as ProRes and then if you are run and gunning it you will be only using P2 which will give you DVCProHD. So you will be dealing with transcoding or rending.
It seems like you really want to buy this IoHD, and it sounds like you have a great deal on it, if you want it get it, at that price you could probably eBay it later and lose no money, It is an extremely handy device that you can use for a lot of things but I would say if you buy it it doesn’t mean you should use it in place it doesn’t make sense, for this job it doesn’t make sense. So buy it, use it when it makes sense.
my 2 cents.