As for booting, when you hit option the "boot" disk is the top of the list. So even if you last booted into Windows, and you have OSX as your boot disk, OSX will be on top and vice versa if you have windows as your boot disk and lastly booted into OSX. As far as I have tried, only Tab works to get to the other OS and then you hit enter. If you happen to have a bootable CD/DVD in, it's always the last in the option (Tab twice if dual booting and want to get into bootable disk, or hold either C or Option C when the drive starts to spin I forget which).
As for helping out, it's what I do and what I like to do. I completely understand how frustrating it can be for a totally blind user to go through this, as you said, there isn't much for help on this topic. Just about everything I learned, I had to do so on my own so there might be easier ways to do things, but the ones I use are all I know. There is one tutorial for Voice Over (but after listening to it and trying things out) I found that it's not totally correct for everything (I'll see if I can find it tomorrow and I'll post it in a reply here). I am a huuuuge proponent of the visually impaired community learning OSX and switching to the Mac. Sure, certain things are different, but most would learn a new OS and screen reader for the convenience of not having to pay up the wazoo for a screen reader, or updates every year. Unfortunately, Microsoft has top dog status in the market and that is what most places use for work. You might also want to look at eBay or Craigslist for the iMac (or even the Mac Pro). They seem to be reasonable priced most of the time (and sometimes even fairly "cheap").
P.S. Another good thing is that you can store your Voice Over settings onto a thumbdrive, plug it into any other mac running at least 10.5 Leopard, and it'll run those settings versus another users settings without messing up the host settings until you take out the thumbdrive.
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VI Tek Talk.
A discussion forum for cross platform, cross device adaptive tech as well as adaptive tech insociety and different cultures.