At first, sorry for my bad English :)
Do you wanna train only to make a profit? If answer is yes, the best choice is buy 18 yo players with potential between 5-8. You can buy them very cheaply (max 100k). You should buy players with high sum of skills. If you want buy guard nice bonus is some inside skills (big man some outside skills).
Level of trainer 4 or 5. It depends on market prices. Of course with low salary (lvl 4 about 20k). You are able to train 3 guys in full training. After 3-4 seasons training you can sell them easily for 1-2M.
About your current 23 yo player. I think it's waste of time train him without trainer. You can't make profit, if you bought him for 2M. It's my opinion. More important is expansion of stadium, it brings you a lot of money in long term.