What do you need? The answer is rather simple – not much. In many cases less is more, less because the coach is forced to use their creativity. When it comes to more, the primary question is need to have or nice to have? Start with a bare room and a chair; add a grass field for other work. Exhaust all the possibilities with those combinations. Improvise; use your imagination and creativity. Olympic champions have been trained in similar environments. Build upon that, add some medicine balls, dumbbells, maybe some stretch cord. Various height boxes for step-ups, sandbags, weight vests, possibly a climbing rope. If you are working with Football, Rugby or the heavy throws then an Olympic bar and weights are necessary.
I have observed programs that use equipment just because they have it. It does not necessarily contribute to the training, but if it costs $3,000, that is a compelling reason to use it. I have seen weight rooms so packed with equipment that you literally had to climb over equipment to get to other equipment. That is not functional from an organizational perspective or a training perspective. Remember less is more, the more tools the less the focus on the athlete. Remember the focus should be on the athlete.