You can only effectively train a few players at once. So, you need to get some developed players, as well as some young guys. I'd sell off one of your trainees, and get a couple of older guys. If you go 30+, they get a lot cheaper - these are the sorts of guys you can buy that will win you up to Div III.
For me, there are two priorities for you right now.
1) Promote at all costs. The higher you go, the better the money - significantly better. The quicker you promote, the quicker your team gets good and your arena gets bigger.
2) Pick 2 guys to be your main trainees, and a couple of others as your backup trainees. Put everything into those main guys, always make sure they get 48 mins per week. There will always be issues with minutes which is why you don't have more than 2 main trainees (at least, if you're doing single position training - which is what I highly recommend).