4 disks is the minimum for Raid 0+1, hence why many smaller sites use Raid 1 with only two disks.
Either way he's going to end up with a single 'un-raided' disk. I think the only way he can use all five disks on raid, is to have 3 on raid 5 and 2 on raid 1, or all on raid 5. Not optimal for a database.
My inclination would be to use 4 disks on 0+1, and leave the other as an on-line spare (or in the draw as an off-line spare).
In my original plan I wanted two smaller servers and I wanted to replicate everything across both.
This got lost in the planning (whilst I was away for a few weeks), but this was kind of okay because we were supposed to have a nas up and running by now, and I was intending to use the nas for the data areas - leaving all five disks for the db etc.
However, the nas is now behind schedule (due to internal wrangling) and my boss has committed us to getting this server up and running before christmas - along with a webserver that is going to connect to it
So i'm kind of left with the server that I didn't really want and not enough disks for comfort.
With regards to the unraided disk, I was thinking we could use it for the offline redo logs - but we may well be better having it as a hot spare.