You can try to sell Sumbing for 1k, but I doubt you'll find a buyer.
Remonde is very nice, even though he is already 19. He won't make as much as an 18yo on the market, but if you're willing to train him, go for it.
Santiago is the hardest choice for me. He could give you some money on the market, but his OD is really bad for a G or SF. If you keep him, you'll need to train OD all season. If he was just a little taller, he could become a great PF, but that runs counter to your current training plans.
My advice would be to keep either Santiago and your two trainees, or Santiago, Remonde and the better one of your trainees, and to single position train them in OD and Passing until Santiago reaches strong OD and respectable Passing. This will take most of this season.