Considering the cost is almost a wash compared to local software, here is something to consider:
Get Google's Postini service: It works very well in my experience. First, you get all the SPAM and virus filtering you need both inbound and outbound. Second, if you do not have a second mail server, it gives you 4 MX records, so if your server goes down, when it comes back up, all the mail gets delivered, and nothing was lost. Lastly, after you have it set up, you can set your firewall to ONLY accept incoming SMTP port 25 traffic from Google's servers, so you face less chances of being attacked by spammers trying to somehow find a hole in your server to relay through.
I would also recommend that you use a reseller rather than going directly to Google, if possible. If you have any setup issues, they are more accessible, and probably know the product well.