1. It depends on a few facts. If the veterans are too old (37 yo or higher) better sell them (or even kick them if they can't be sold). I would keep them if they're still "young", or at least they are still useful. It also depends of your own goals next season.
2. If you are going to prepare an unipositional training plan (OD, IS…), 3 players. Otherwise, it is recommended to have at least 6 trainees for bipositional training plan (such as one-on-one).
3. One-on-one for SF/PF is usually the best choice for starting training.
4. No idea, that's too subjective… it's not the same at all for a IV division than for a II division team. You can still find some good but old players. The one that are up to 30-32 yo are still quite expensive.
Hope it helps a bit, good luck
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