Just get the prepaid ones. Buying sim cards, IMHO, are for those people living in countries where they've put restrictions on the number of sim cards a person can get.
If you can buy prepaid ones, that's your best bet.
I've read the thread of the seller, saying that you need to recharge a sim card to stay valid and he's right. However, a sim card, without recharging it, will not expire for at least 3 months minimum. Other providers give longer time periods.
Instead of buying pricy ones from another country, get yourself those .99 cent ones from Ebay and go crazy. It's not a bad price.