You can enable it later, but enabling it won't magically convert your root partition into an LVM volume. If you install on LVM, you'll have LVM controlling the OS partitions as well as anything you add later; if you don't, it'll only be things you add later that are LVM'ed.
The same is generally true of RAID - if you want the OS to live on RAID, it's a lot easier to set up at install time (cute as the linked question thing is, they're not really two different questions; or at least, they could very easily be handled in the same posting).
Having the OS on RAID-1 is a no-brainer, for me; I don't know why anyone wouldn't do it, comparing the cost of hard drives with the cost of admins and downtime when a drive fails. Note that RAID-1, although excellent, is not a substitute for backups.
As for whether having the OS on LVM is the right thing to do, that's more of a judgement call / matter of opinion. My personal feeling is that if the machine is going to be stable after installation - it's a fire-and-forget machine, where you'll never want to resize a disc or re-partition any of the volumes - then there's little point in the added complexity of LVM. If, however, this machine might have a more interesting life, then LVM can be of very material assistance when you're resizing / moving / adding drives later on.