Important features (in order of importance)
- large qwerty keyboard for txting (they find T9 txt a bit tricky) ; might be ok with qwerty onscreen keybd.
- easy to read screen (they are old)
- battery life (they often forget to charge the phone)
- low price (they don't use cellphone often)
- simple menus (they are old)
I was thinking of the Nokia e75 - $330 @ Eking / $380 @ Mobicity
I also don't mind buying second hand if it was relatively good condition. Another important thing is battery life as they often forget to charge the phone. I've used nokia in the past and quite like that the menus are simple, but not sure about the newer nokia's with more functions. Would prefer to spend <$300. I also thought about Vodafone 845 for $199, but not sure how they will cope with on screen resistive keypad.
They lost their last phone, so doesn't matter if it goes on telecom or vodafone prepay.

LG GS290
Nokia 5230
LG GW525