a balance between arena, modest trainees and modest trainer will be all the $$$ investment you need to secure an increase in long term income and performance on court.
in my time as manager so far i have had less injuries than some other managers. this may be down to luck, or perhaps it is down to them playing their players 100+ minutes a week and breaking them.
aside from this i did notice the number of injuries in my roster fall with a higher level doctor.
i think i got a level 3 cheap with low salary (and massage speciality) after i chose my trainer/trainees and started building arena seats.
i now have a level 5 with massage and ive just had my first injury in 2 seasons.
in future i will stick with level 5 for the peace of mind and knowing my trainees might get that extra week or two of training as a result of good health