Ive done this, but with a slightly different configuration--But in theory, its all the same.

If you want a absolutely no issues setup like this, would follow the above instructions and just not add a 2nd server.  Do keep in mind that if network load balancer isn't set to be auto started, you need to start it every time you restart your mail server. You might be able to do the failover without a shared IP, but at that point your so close to setting yourself up to have a redundant server that you might as well do the ground work now, instead of later.

Also, when you update smartermail, during the install process it will stall, saying it couldn't start the smartermail service. When this occurs, go to the services panel, find the smartermail service (if its still trying to start, kill the mail service with task manager), right click on the smartermail mail service, > Properties > "Log on" tab > and give it domain credentials that work on both the local machine and the NAS folder you have configured.

Also, please keep in mind the IO loss you will likely experience via going to NAS, and make sure you have a decently good raid array or are using good speedy disks.