You don't know if he's going to be a good performer or not. Your best bet is to look at his current club record and see how well he is doing. If he doesn't have a club well... you just have to chance it.

Question 2 is too vague, you can score with any formation against a far superior team if you set up your orders right. Generally you should play attacking since if you defend you'll probably concede and not attack.

Don't worry about defence. You'll always concede somehow, the game is made to frustrate you lol