Hi Suki
Your situation is quite complex and I would advise you to seek helpfrom a Welfare Rights Organisation; you can find your local Welfare Rights Organisation to help you with advice by clicking on the link below and putting in your post code, it will then come up with a MAP with a list of agencies.
advicelocal.uk You are quite correct when you say that when you reach State Pension age your ESA will stop but your PIP will carry on. Your husbands CA will continue as to whether or not you would be eligible to claim UC which is a means tested benefit would depend on your house hold income and whether or not you need to claim the housing element. This is why I would advise you to contact your local Welfare Rights Organisation.
Gary