Try and train 18 year olds while you can, by 21-22 they slow down a lot.
Also, the difference between a basic trainer (cheap) and a superior one (expensive) isn't that much. My guess is about 25%. Since you're new, I wouldn't get higher than an advanced trainer. That superior guy must be eating up a ton of your weekly expenses and when you're in a lower division, you can't really afford that.