I think as long as you get value for your 500k, it doesn't matter a ton how you divide it up. Particularly if he's selling one of several trainees at a position where he already has a lot of talent and using the money to buy an upgrade at a non-training position, I think it works out at least as well as trying to cover several positions and possibly spreading your money too thin.
It depends a lot on your individual league, but I don't know that you can really get more than 1 'impact' player for 500k anyway, unless you're sinking it into older players that will need to be replaced sooner than desired.