Hi,
You can buy 1st Generation mobile and you can also unlock your mobile,But you can use Verizon in your mobile if it is previously locked to any CDMA network Because Verizon is a CDMA Network.otherwise you can't use Verizon,you can use any GSM network.So You can Check whether it is locked to GSM or CDMA.If it's locked to CDMA network.You can buy that mobile and unlocked it by Remote unlocking in the site http://www.wickedunlock.com/ .They are cheap and reliable.I unlocked my iPhone in this site.So I recommend you to view this site.It will be very Helpful to you....