hello and welcome to the site. Thank you for your question.As your mother is selling the property to a connected person (i.e. her son) for capital gains purposes the selling price would be the market value of the property and not the discounted price you pay her. You may find this article helpful . see link here
In your example, £50k would be a gift to you and for inheritance tax purposes regarded as a potentially exempt transfer and the seven years rule would apply.
If your mother survives for 7 years after making a gift to someone, the gift is generally exempt from Inheritance Tax, no matter what the value.
More information on this can be found here
Recipient of the gift receives the money free of tax. So you would not pay any direct tax on the gift of £50k on receipt of it.
I hope this is helpful and answers your question.
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