Just because you want to train Roy does not mean that you can't train other players, too. If you're one position training him then you can get 48 minutes for two other players. Buy more players at the same position as him, and then either keep them around or sell them to buy more, better players. This will not have a negative effect on Roy's training.
If you're hoenstly trying to turn him into a NT level shooting guard then you should definitely buy other trainees to train alongside him.
re: Arena
You improperly invested in it. Your current arena situation is a waste but you can redeem it.
What you're asking depends on what you want to do. If he's getting a lot of time at PG then there's no point in buying an older PG, but if he's getting a lot of Jump Range training then it might make sense to get him a guard.
This also depends on what you want to do in the long term. If the goal is to try to make Roy a scorer then getting him some defensive, rebounding and PG help should be the priority.
My suggestion would be to get an older (24-29 year old) centre with strong or proficients ID and rebounding, and decent secondaries (particularly passing) so that he doesn't end up taking a lot of shots. I would also invest in some good defenders at the other spots, and in the long term look at getting a defensive/pass first PG.