We moved DD1 when she was 8 months and again at just 3.
1st time nothing matter the most important thing in the world is you, her mum, she couldn't careless of where she was as long as you don't change her "routine" (loose sense of the word) too much.
At 3, she did missed her friends (yes, she had meaningful relationships), and was angry not to be able to talk to children in the park which made us chose a local school instead of an international one. She is very outgoing though and make friends easily, the 1st few months were touch but it got better with the new year in september as new children had arrived too.
If you can move during summer holidays and be there for the start of the school year it will help older children.
We moved her whole bedroom so she could feel at home. I think it helped seeing other parents who only took a suitcase/kids. We had far less tantrums at home because she could retire in her own world. (but we are pretty minimal and she doesn't have many toys compared with most of her UK friends).
If you want another kid you have to consider what happen if you want/have to go back to the UK to have the new baby (do you have a place to go back to?, DH will probably miss the birth, DC1 out of school for a few months....)
But as TL said it all depends where and the setting you will have once there.
Don't take your children as an excuse not to do it though, they are adaptable, if they weren't they wouldn't grow up, and it is a good life experience for them.
The adults are usually the less adaptable and problematic part of the family IMHO and IME, and the children might pick on that or copy it.
Good luck. And feel free to ask more questions.