Build a Web based Email Service

I want to build a web based email service where i will have a database of email addresses segregated by industry type, company name, designation, department, city, country and other relevant information. It will be a paid service and my client will not be able to see the email address. Instead they will be able to select the various filters (industry, department, designation, city, country etc) and send them an email directly. After sending the email, they should be able to see a report with the details of the email sent, opened, clicks segregated by industry and other filters. Email server to be used should is Elastic mail. The receiver will see the email address of the sender and will be able to reply to him directly. These are basic requirements but there will be more changes and revisions later, and i want to hire a long term consultant who will be able to modify this service as per my customers demands. I would need your basic project cost for development and then per modification or revision cost as time progresses.