Your eldest daughter is eligible to act as an executor and/or a guardian. As Theas says, being appointed as a guardian doesn't necessarily mean that she'd have to have the younger two living with her, but would be able to make appropriate arrangements.
She would be able to hire a solicitor to help with the paperwork involved in acting as an executor.
I'm a will writer and normally recommend that you don't have the same people in both roles, partly because it's a lot of responsibility to look after bereaved children and do all the running around of obtaining probate, but if you are very short of options and trust her with the money then you could appoint her in both roles.
You need also to have a second default executor as a trust would automatically arise if your younger two were under 18. The second executor would act as a gatekeeper to the trust to prevent any mismanagement of the fund.
So far as your estate is concerned, money held in pension/insurance/death in service trusts isn't counted in for inheritance tax; the tax is only calculated on your estate, which comprises you house, contents, cars, savings, shares etc etc.
I'm a will writer and if you're interested, I have a paid for advert "5* Will Writers Recommended by Mumsnetters" over on the Small Business Section of Mumsnet Classifieds. 