Some of the comments on this thread are daft. OP - treat them with caution. For that matter, treat my post with caution, as you have no idea if I'm also a layperson making up random laws to suit my political beliefs. 🙄
SDLT is paid on the chargeable consideration. Usually, an outstanding mortgage catches you out because you're taking on the liability for that debt, which is considered chargeable consideration, even though no money is changing hands.
Your mum could gift you her property with its value of £850k. As you say, if she survived seven years, there wouldn't be any IHT implications on that gift. Please note, though, as @VeggieSalsa correctly points out, there is a massive difference between an actual outright gift and a sale with deferred consideration (i.e. where you pay over the cash at a later date).
If a true gift... your mum would be down an asset of £850k. This could cause issues if she needed the £850k. Her circumstances could change.
Your subsequent £650k would be a gift to your mum rather than a contractual obligation. What would happen if 1) you couldn't sell your property, 2) you fell out with her mum, or 3) you died?
You and your mum may be happy that 1) and 2) are low-risk. You should make sure you have a will in place to protect her from 3), but given your house is worth £650k, as a ballpark, she would only get, say, £520k less legal fees. That then becomes a hit of £330k from the original value of £850k. There could also be a long wait before the estate paid out, and she would not only be getting a reduced sum but there would be that timing disadvantage.
This feels like a really risky transaction for your mum.
If she simply sold her property to you for £650k, yes, there would be SDLT of £20k that you'd have to pay, and she'd only get £630k (your net proceeds), but she would be much safer. It would also be much less hassle for everyone.
I suppose you could take out extra life insurance on yourself with your mum as the beneficiary, so if you died whilst this debacle was going on, she wouldn't massively lose out on the sales proceeds.
It does feel like a lot of hassle just to protect £20k, though. I know £20k seems like a lot, but when you factor in paying for the legal and tax advice you should get before entering into this transaction and consider the hassle and risk (which will especially affect your mum)... I'm not sure this is the brilliant idea you thought it was.