Using equity release on property to fund first time buyer deposit

Hi all,

I was left 50% of the family home when my mother died 7 years ago. My
father still lives in the home and we get on very well and he wants to help me get on the housing ladder. He is too old to use a 'help a hand' mortgage (73). He also has other investment properties but I do not have any % interest in these. He may move out of the property in 2-3 years.

The home has been valued at £250k and there is no mortgage. Therefore there is £125k of equity that I could use.

I want to raise some cash to raise a deposit to buy a flat in London where I live. I have saved £30k and income (w partner) of £45k per year. 4*45k=180+30 = £210k borrowing no problem.

Ideally we'd like a £250-300k house, so a rough calculation leaves us 60-80k short.

We would like to use some of the equity in the house I inherited to raise a bigger deposit for a flat. I want to take out an interest only loan
of 70-80k against the equity and then use this to put down a £100k deposit

1) How much would you think was realistic to borrow against the equity
2) Any other advice?
3) Do you see any potential pitfalls in this idea?

Thanks very much for reading.

Bests,
R