I don't know anything about equity release but this is what I know about buy to let.
It's very unlikely that you would get a buy to let mortgage. The amount you want to borrow against the value of the house is too high. Also your age might count against you.
You could raise a mortgage agai St your current property or possibly increase your current mortgage and use that.
You would have to pay the extra 3% stamp tax because you would be buying a second property.
Contrary to what the media might have you believe there are not vast profits in letting and many legal obligations. Taking into account mortgage payments, tax, safety checks, legal responsibilities, upkeep and maintenance plus perhaps agents fees, I think you would find that you may even make a yearly loss. It needs some careful calculation. You need also to factor on the damage that a bad tenant can do. That can be thousands if, for instance, the kitchen needs replacing. The deposit won't cover it.
I'm not saying don't. I'm just saying it's more complex than you might think?