Border Collies need room, and have to work. They sure ain't house dogs. Damn fine dogs at the farm, though.
If both parents of this dog are mixes, it isn't any more Border Collie than I am, really. If there's a good amount of Lab worked in there, you're going to have a whole different dog. If it's got a bunch of German Shepherd mixed in, there you're gonna have a little less strung working dog.
Best you can do is go visit the little buddy, take him home and give him a try. If he's high strung, you're not going to get it out of him. He'll bounce and run till he drops dead.
You mentioned you've been enamoured by Jack Russells, but have you ever worked with one? They're nuts, maybe even more strung than Borders.
I guess I just wanted to show you asked a big question.
Takes me right back to "Go see the little guy and try him out." That's the only real way you're going to guage "who he is". Have fun, Marcus. I'm jealous!!!