It's a good guess that it's a staffy mix. I get this from the eyes the ears and the brindle coat. She may fill out a little more but she's too slight to be a full bread. If I had to guess (and I obviously do) then I would say that she could be mixed with a lab. In the picture is a cross between a staffy and a wippet (of all things). Maybe that will give you some ideas for further research.