I think you should stick to 0.1 btc. I would buy at least 100 of them if not more. And you should preload them all. Doesn't make sense for people to load them themselves.
And please put on your website information for people who received these coins and just googled casascius.
This.
Is there a website that explains in detail how one actually redeems the BTC from a coin? Somewhere we can refer coin recipients to?
+10
Do you mean something more detailed than go to MtGox, click "Add Funds", and choose "Redeem Private Key"?
The alternative involves patching source code and is a billion times more difficult. It's out of reach for the average user.
StrongCoin has been mentioned already. pywallet might be another option.