Roadmap to 24x7x365 availability?

I need to plot a path to achieve as close to 24x7x365 availability as
possible without breaking the bank which means I may have to take several
incremental steps before reaching the goal. My current environment consists
of a Power7 with 3 of 6 cores activated as the production box and a dual
core Power6 box for test/development. Both are running 6.1 and use internal
disk. Both boxes are single partition but I do have an HMC and the Power7
box is licensed for PowerVM.

We are running a commercial ERP and will be starting a project to develop
some new apps using PHP and Zend Server. A whitepaper I read states that
IBM has tested the ERP and it is capable of running in an iASP.

My immediate needs are to update the ERP system to the latest release level
within the current version and to upgrade the OS to 7.1 without incurring
significant downtime. Eventually, I will need to perform the backups, PTF
installs and other maintenance with near-zero downtime.

I've been reading about PowerHA SystemMirror as well as combing the
archives for posts on PowerHA. Based on what I've read, I think it is
probably a good solution. However, I would have to start with internal
disks since a SAN is cost prohibitive at this point. So would
creating "Switched iASPs" be a good starting point and is there a map
somewhere to get me started down this road?

Thanks in advance for your insights and suggestions.

BJ