Contrary to what some may say, many companies have successfully migrated their mail and applications from Lotus Notes to SharePoint. There is a proven process to do so. Your management will have to decide on the exact steps to take. Time and budget will help define the level of effort required.
I have been involved with Notes migrations in several different roles for 10 years now. I am now the Product Manager for what I consider to be the best Notes application migration tool on the market: Notes Migrator for SharePoint. I will gladly put you in touch with one of our migration experts. Then you can decide for yourself what will work the best for you.
http://en.community.dell.com/techcenter/sharepoint-for-all/f/4800.aspx
http://software.dell.com/company/contact-us.aspx
You can also look at some of my blog posts on this topic.