I am going to answer it point wise as you have asked:
1. Approximately it might take about 1/2 an hour to re-design each page depending on any number number of graphics you might have to design for each page. More the number of graphics, more time its going to take.
2. Usually webdesigners charge at an hourly rate. Fix your hourly rate and charge accordingly.
3. Google rankings depend upon a number of factors:
3.a Your page should have content accessible by search engines
3.b The content should be based on high ranking keywords
3.c Proper meta elements should be there
4. I believe it would be far much better if you convert the entire site in Joomla. Since barring a few pages, the whole site would have a similar look and feel, you may create a particular template and render those pages with it.