I would be very hesitant before I started outsourcing your email through Google- it can be a very unnecessary expense. Plus, Google isn't going to like (or handle) mass mailing applications.
For regular email, I would keep that at your mail server. How many thousand emails/day do you expect to get (incoming)?
For bulk email, you can either use your existing domain name, or register YourDomain-Mailing.com, and use that strictly for bulk mailing applications. If you go that route, you can also easily use a VPS server (think $20-$30/month), and use that server strictly for your bulk emails.
Email servers are, for the most part, very low in resource usage. I would consider hosting the mass mailing server with an entirely different provider (think cheap), mainly to protect your main mail server from being associated with spam.