Nothing to stop the sole beneficiary being the executrix [female] or executor [male] of their parent's will. They can't be a witness to it, but they can be the beneficiary. It is very common for that to be so; I was executor to both my parents' wills and the sole beneficiary to the second.
I can guarantee that you will need professional help,if only to understand how the created trusts work. There will be an enormous amount of correspondence to deal with,mostly from the Revenue, and in connection with any business or property. I had nothing much else to do, but still found it daunting and I needed professional advisers, paid from the Estate, at every turn. If you feel confident in dealing with them and keep a kindly eye on their charges, well take it on. And my mother's estate has taken 4 years to wind up.