mass mail setup for multiple clients

We are looking to offer mass mailing as a service to our clients. We need someone to advise us the virtual server and physical server setup needed then setup the software and get it in 100% working order. We are open to all suggestions and guidance on which software to use at this time we are looking at Oempro frontend with postfix backend for cost reasons. For amount of mail to send we are just starting the service and want to again build this in a virtual environment so it can grow. We will follow your advice on this as well. The client we have has a 35,000 person list but it does not have to go out same day. We do however want to of course build this to allow a large amount of mail at the same time without problems. Time line is important to us to please put the time you estimate this to take and be sure in your ETA.