what kind of coding is involved in SEO job?

[MALCOLM]

SEO generally doesn't need any knowledge of coding if the website has a WYSIWYG content management system. If you have to rely on changing the code of the website manually then you'll have to have knowledge in the language in which the site is written.
WYSIWYG is an interface like MS Word where you can add things like anchor text for links and alt tags for images.
Depending on the site's construction you may or may not be able to edit the meta data (title, description & keywords) of the site as some sites are very basic and only allow one set of meta data to be used in which case you are sunk as the site will suffer from all pages looking to the search engines and so will be penalised for duplicate content.
As far as the responsibilities of your SEO consultant then you'd have to decide these at the outset. For example who will be responsible for uploading the SEO content to the site and when invoicing is to start ie do you pay from commencement of the agreement or when the site has achieved a certain position in search results ranking.
You'll also have to agree (as the site owner) to any changes in text that are required.