CGT, Inheritance Tax, Stamp Duty - help needed!

[busylady1]

This is rather complicated but here goes

My Mother died 2 years ago leaving all assets to my Father. He therefore now has a IHT allowance of £650k is my understanding, and for simplicities sake his estate is unlikely to exceed this.
He has one property (the one he has lived in for the last 9 years). He is considering selling this, and moving in with my sister. He will give her £200k towards a new property to accomodate him, and give us £200k leaving himself circa £100k from the property to live on (along with his savings, pension etc).
My question is this - we are quite interested in buying the house he lives in as opposed to taking the money, but are concerned about taxation on this. What are the implications of tax ie stamp and CG if we do this. I have tried to research but I can't work through if CGT would be due on the value of the property when transfered or the value of the property when he bought it?? There will be no mortgage on the property if we purchase
The scenarios would be
a) He gifts us £200k and we buy the property for £300k (would we have to pay stamp duty on the £300k and when we sold the property would we have to pay CGT on anything over the £300k
b) He gifts us the property in its entirety and we gift his sister £300k (no stamp duty I presume, but what would be the value the CGT was calculated on??
Any help would be appreciated. Will post on the Inheritance Tax forum also. Thanks