A lot depends on the company you are using
When we did our one, they took the old one out one morning and came back and put it in over the next couple of days, including plumbing and basic electrics. Anythng fanct like downlighters unless they are supplying it may need to be done by yourself
You more then likely will need to plaster the kitchen afterwards and depending on what your lkitchen is like, may need to retile
When you are talking to the designer/company, you should ask exactly what they will or won't do and ask for a breakdown of costs,