if you go to iTunes App store you can sign in and then if you click on your ID next to the amount of money or credit you have, there is account ,redeem, my wish list ,sign out .Go to account you may have to re sign in go to purchase history you can see all you purchased and then ,Above that section you have..
iTunes in the cloud then Hidden purchases you will see a phrase
unhide any hidden purchases,redownlodable purchases
then click on MANAGE undide any App you purchased on that Mac. com, Mobile me ,icloud account.
some Apps are not compatible with recent iOS and iPad only or both so they will crash the app may have a new version and you may have to buy it again.