Client growing and needing 99.99% email uptime

Have small business client with only 25 -50 employees that has been growing for last few years and now is getting million dollar contracts. There only communication between them and their clients are through email so it is now crucial to be up 99.999% time and have some type of redundancy in place. For years they have had 1 wan and 1 box - currently SBS 2011 running MS Exchange 2010 /, file server, printer server, etc and runs well. But there have been times due to email issues or other features on the box where had to be down for short time. That is OK for the other features but not for email. There are 2 routes I could suggest ... pay bookoos for second wan, second box (physical or virtual) and have 2 Exchange boxes in sync, plus the cost of antivirus, antispam and maintaining the growing network and issues that arise. Or should I just recommend they go with an exchange hosting company like office.com. They only need like 20 emails for client communication which would be 20 x $48 = $960/ year. For the other 30 staff employees who are all like grounds-maintenance people, just have them use an outlook.com account. The client would literally save thousands each year on IT issues and obtain their 99.99% uptime and redundancy. The only possible "con" is when there are email issues, what kind of support would I get from office.com or maybe godaddy.com? I am not concerned with losing business, just what is best for my client... Just asking for recommendations on how you would handle these growing pains.... thx.