"so is there any specific criteria for choosing the WAN link type & specific bandwidth when we want to terminate the WAN link on a switch."
Not sure i follow. As long as the link is ethernet you choose the bandwidth.
"f we want to terminate the link on a layer 3 switch...is there any extra equipment or converter required ??"
Again, it needs to be presented as ethernet and you will be fine. Obviously if they present fibre you need a fibre connection on your switch. If the WAN termination is anything other than ethernet you would need some sort of intermediate device such as a router.