Yes, I would train a guy with 3k if he had enough potential for the AA team, actually I'm starting to train a HOF with 4.4k guard that only has this salary because of the rebounding.
I think you should NEVER train a player with salary less than 3500!!
So, even if he reaches the II in some year, he shouldn't train a big potential player with less than 3500, with the objective to take him to the National Team?
With a level 4 trainer you could cap an allstar player with good starting skills in 3 seasons.
I wasn't talking about him, of course that in this moment he doesn't have much money, but it's a bit weird to say that you shouldn't ever train a guy with less than 3.5k despite having good potential.