In your place, I would most definitely not make my step-daughters my executors. And forget about a letter of wishes. Anything you want to decide about how money, keepsakes etc. are to be bequeathed needs to be written down in your will, which should be drawn up by a solicitor. He or she should be able to estimate what the charges will be for carrying out your instructions.
You say your own daughter has learning difficulties - rather vague, if she is under any form of guardianship this needs to be addressed in your will too. I would not trust sisters, stepsisters or any other relation to see fair play, as even in the nicest families people tend to change when inheritance is at stake. If your daughter cannot deal with her affairs herself, you need to appoint someone to deal with them for her.
The executor's fee comes out of your total estate before it is apportioned to those you leave it to, which seems to me to be the fairest way of dealing with the matter.