id consider training 3 or 6 guys you start with while you take a look at the market for 18-21 year olds and see what is valuable and what is not. id go as low as training a "star" if they start on your team and are under 21.
you dont necessarily need to spend money on good looking 18 year olds to make money if that is your aim for now. its possible to buy a player (say 19 or 20 years old) with a low salary for 50k, train them for a season and sell them on for 400k+/-. while you do that you can get to grips with your finances and start investing in your arena.
i personally think 18 year olds are far too overpriced and that you can get some great 19 and 20 year olds to add value to.
if you want to train a player to keep for a while, again star potential is fine. My player Bukus was the first trainee i bought and is still with me as a bench player now.
scouting is a lottery and even when you think you have a winning draftee on paper, they turn up and they have holes in their game that will take a season to fix and that will make you want to cry! if you are coming in late consider saving any scouting points for next season or better still the cash to buy a draftee you can see properly!