The stock Grizzly tires are only 4 ply and add a lot to the instability of the stock machine. There are a ton of tires to choose from, but what you want is a 6 ply tire. After that it goes by the type of riding you do, what catches your eye and your price range. I did a lot of research before I bought mine and I chose the Dirt Devils because they are a good wearing tire, they have excelant traction in mud and soft dirt and they are a good smooth trail tire not to mention that they are one of the cheaper tires on the market. If you buy aftermarket wheels the off-set will give you a little wider stance which will make the machine more stable when cornering or on tricky side hill situations. Again, there are a lot of wheels to choose from and the price for a particular wheel can vary depending on where you buy them. You have to shop.
The stock air filter is very poorly designed. It allows dust to go between the filter and the air box. With the Pro Design set-up, you bolt an aluminum flange to your air box then you can put either a foam or K&N filter on that securley mounts to the flange and it won't allow dirt to get to your motor. Also, by puting an aftermarket air filter on you will be increasing your airflow which makes your motor run a little more efficiently. Later on if you want to add an aftermarket pipe you will get the most out of your stock motor but you will probably have to re-jet the carberator.