Without taking a look at your roster, I'd say get 1 more trainee if you're training 1 position/week. Training is the first priority to have filled on your roster. You can get by spending much less money than that on a quality trainee, though. So with the remaining money after you get the last trainee, upgrade your arena and then look to upgrade non-training positions on your roster if you need.
Don't spend money without knowing exactly what the benefit is.. aka if you're already going to win your division this season with the current roster there's absolutely no reason to spend money upgrading it right now. Alternatively if there's no purchase you can make with X amout of money that will allow you to win the division/avoid relegation/whatever your goal is currently, save up until you're able to make a difference. Having cash on hand never goes out of style!