To buy everything in the store, you must first unlock everything which will come naturally as you level up. You will then need to amass enough money to actually buy everything, which will take more time than hitting level MAX. You are best off buying things that will benefit you first, since some of the equipment will give you bonuses to certain stats, like strength or balance, allowing you to complete levels a little easier. Don't bother with equipment that helps your swimming, since that only decreases the penalty for failing by half a second. This will probably be the last achievement you get. The easiest way to grind is to play the Winter Sports episode (Episode 3) repeatedly, since it can be completed very quickly and you will be given a $50,000 bonus for completing the full episode, giving you $58,000 every 6 or 7 minutes.