As for the 505's battery, it may take a while until an OEM battery becomes available for the 300. But I think there should be something on the market by the time you need one.
I have used my 505 quite extensively, and after more than a year it's still running around the same charging cycle as when I bought it.
On the other side, I've upgraded to the 600. I know that there are people who use a unit until it breaks down, but I'm more the "I want a new toy" sort of guy (as far as I can afford).
