Are there any items that you NEED to pay for in order to get?
Doesn't seem like it. Seed slots so far is limited to 5 slots and seems way to small, so it's hard to get too imaginative with your layouts so far. Which I have to say I really do not like, but there is a much smaller group of plants to pick from anyway (at least so far). I know you can buy more slots for real money, but I am hoping you can also unlock them in the game, but I don't know if that is possible or not yet.
This game reminds me of Flight Control. It was a nice simple game, but well done and incredibly addicting. EA bought Fire Mint and made FC2, trying to make it much more complicated and get you to buy IAP. While number 2 wasn't bad, it certainly missed the magic of the first one. The same goes for PvZ2 for me. It doesn't have the magic of the first game, but as I said, that may because I am burnt out on it.