I don't know of anything that will give you anything even close to Gmail as far as an interface/search/chat. Also, running your own email server is basically saying "sign me up for an endless source of pain". At my current job, when we were hosting our own email, I was spending 4-5 hours a month on email issues on average. Google Apps is either free or $50/user/year - although it is a bit less flexible, it is well worth it to me.