Yes, you need antivirus on your mail server. You can outsource scanning inbound and outbound mail for viruses and other malware (as well as spam) to a number of other companies, and I recommend this approach, but even if you do, your mail server is still exposed to the threat of becoming infected by all the internal clients that you have connecting to it.
At a minimum, you need AV on your mail server to protect your mail server, and hosted solutions don't protect it from your users and client machines. Whether you use the AV to scan messages, and for what, depends on whether you have a hosted company or appliance at the gateway doing it for you. If you've got something else doing it, you may be able to skip this step, or use a less intensive scan, if you don't, you definitely need to scan emails thoroughly.
And yes, there will be a performance hit, but it's generally pretty minimal, so unless your mail server is already being crushed, adding AV shouldn't cause performance problems. However, this is (one of the reasons) why you'll want to trial different solutions from different vendors before settling on one, and one of the reasons AV vendors offer demos and trial versions for enterprise customers.