Captalised spousal maintenance

I am having an issue in negotiations about capitalising spousal maintenance. My ex is requesting a lump sum payment of £75000, to cover monthly maintenance of £650/ month up to retirement age. My main problem is I cannot afford this. The key issue is that she doesn't work but has done in the past, they are on UC of circa £800/ month with PIP of £350/ month, living in supported housing due to substance use issues, although it might be worth noting she was recently threatened with eviction due a significant relapse. My thoughts, and they might be incorrect are these;

1. If I cannot afford capitalised maintenance, then any order should be for periodic maintenance, if I pay 12 years of maintenance in advance I would potentially be out of pocket because she could find a job, new partner and will inherit a large sum on the death of her parents (both in their late 80's and in poor health).
2. If I pay periodic maintenance of £650/ month this will just be deducted from UC payments so there is no advantage, even payment of a lump sum would mean UC is no longer available, the alternative would be for me to pay the equivalent of UN (800) and maintenance (650), a sum that would account for about 80% of my net income after bills.
3. I have agreed to purchase her equity in the family home where I reside with my two children (age 18 in full-time education and 20), paying an additional 75k would mean I would not be able to fund the mortgage. The purchase of equity provides enough for her to purchase a new home outright and pay of any debts.
4. We have agreed a pension sharing order, after advice from an expert that would provide a significant pension, from today of circa 28k per year, but she believes she doesn't have to rely on pension income until of pension age.

I thought maintenance was less about parity of income and more about meeting needs, so after divorce she has the potential to realise an income of circa 2.5k/ month with no mortgage or rent to pay and this is sufficient to meet the needs of a single person.

All your numbered points are valid. I doubt that any court would compel you to pay capitalised maintenance under these circumstances so if you can't afford to pay it and you don't want to (for the good reasons that you have given) then just say, 'No'.

Many thanks for your wisdom in the past few months. Final order came through today after consent order sealed a week ago, happy with what was agreed and the advice of my solicitor who essentially had the mantra "agree what you can agree and if you can't, involve me".