lesnar isn't a good matchup for Orton IMO. 2 reasons.
Brocks physicality and Orton has had a history of injurys
And Orton I still think will turn heel soon. Heel vs heel doesn't work
Orton's injuries come from his shoulders, though. And he's also sensitive to concussions. But I don't see it as a big deal. Orton is still young and the last time he had a real shoulder injury was at Over the Limit when he pounded the mat way too hard in the match with Edge. He still recovered pretty fast. As long as he doesn't get dumped hard on them (like ONS 08 with HHH), he'll be fine.
As for the heel turn, it's all speculation for now. Maybe they'll hold on with the turn for a few months longer just to make this match happen. And as much as I want heel Orton, I'm perfectly fine with keeping him face a bit longer just for this match.
Lameus making fun of Lesnar would be embarrassing to watch. Face Sheamus is awful. He doesn't take anything seriously and I just can't stand him. "Hey fella you won one match but this isn't over yet next time I'll kick your arse" Maybe heel Sheamus vs face Lesnar would be a better idea