My mates with the os 70 fs is only about a month old put has been abused in the air about as much as you can with no sign of failure so far. I also knocked the undercarriage of it on it's first day after losing it in a low harrier and no other damage occurred. The wood does look a little odd but all I can tell you is it has taken it so far.

Engine choice, a lot of 3Ders swear by four stroke for it's low down torque and throttle response, usually I think this means going for bigger size for the same power out put. I'm not that experienced but from what I have seen I don't think you could go much bigger than a 70 four stroke for it's physical size and can't see you needing more than a 50 2stroke, it may fit but I wouldn't think you will need it. Hope this helps