Thanks for your reply
If your parents wanted to go on the Deeds, this would mean they would also have to be party to your Mortgage. I am not sure whether your Mortgage Lender would agree to this, bearing in mind your parents potential ages, but they may do. If youy are able to proceed on this basis, you can all hold the property as "Tenants in common" meaning that you all can agree on what percentage shares you have in the property.
Alternatively, if only you/your wife apply for the Mortgage, then only your names can appear on the Deeds. In this scenario, provided your parents were prepared to sign a simple letter stating that their contribution towards the purchas eprice was a gift, and is not repayable, your Mortgage Lender will be happy to proceed on this basis.
(Your parents contributing to the price is only an issue as far as your Mortgage Lender is concerned, IF your parents were wishing some sort of Charge registered against the property, in order to protect their financial interest).
As rgeards IHT, both your parents have a joint tax free estate of £650,000.Therefore, normally, if they were to make a gift to you, and then survived for 7 years, the amount of the gift would not be classed as forming part of their Estate when calculating its value.
However, if they were to live in the property with you, their gift would be classed as a "gift with a reservation" (ie one whereby they are reserving a benefit), and such gifts are classed as a "scam" by HMRC, in that the normal 7 year rule will NOT apply, and your parents will still be treated as owning the amount gifted, when calculating the value of their Estates upon their death. In simple terms, therefore, they would be treated as still owning the £600 odd thousand they give to you even if they were to die in 20 years.
I hope this assists and gives you the legal overview. No doubt the Solicitor who acts for you will be able to confirmt he above.