Lvl 5 trainer is not needed in div V, so stick to lvl 4 trainer. You should invest a lot of your earning into expanding your arena. If you expand now, you can focus on getting better players later on and this is more efficient (you don't want to get better players, then expand arena, because your weekly net income will be low. if you expand your arena first, the weekly net income will be high and you can quickly earn enough income to buy better players).