I've made some changes, Chen Zhe, but perhaps the Smiths would like to hear your ideas instead of mine, expressed as you would express them.
It's your choice of course.
Dear Mr. & Mrs. Smith,
I am writing to invite you and your wife to Wuhan.
There is a long time we have not seen each other. It has been a long time since we have seen each other.
I remember you visited Wuhan five years ago. And it has changed a lot. If you come, I will show you around the city, especially the new [founded] art museum. I am sure you [must] will
be interested in it.
We hope you can come. We are looking forward to seeing you. Just to make sure things will happen as expected please write back as soon as possible. Then I will arrange things reasonably. Then I will make all the necessary arrangments.