Okay, I'll be brutally honest. If you are new to Sitecore, you should recommend that your client hire somebody else to do this, or you should partner with someone who has more Sitecore experience than you do. Otherwise both you and your client are in for a hard ride through the long night.
And just for a few of the gory details: you likely be transitioning Lucene to Solr, WFFM to Experience forms, recompiling custom code against 9.x and fixing anywhere the springs pop out, and as Rohan pointed out possibly porting over xDB. None of these tasks are for the faint hearted and, on some sites, can be far reaching and labour-intensive. Does the site use a lot of forms? Watch out for the (missing) save actions in 9.x. A simple WFFM with email save action and captcha might take you quite some time to redevelop in MVC.
Sounds like a helluva lot of work to get additional sitemap features.