Ok, here how the system works.
For every 450 credits on the humans load-out, an extra evolve is distributed, for a max of 3 evos.
0-449 = 1 evo
450-899 = 2 evo
900+ = 3 evo
Now, on the other idea, make a 6bp building called an ammo rack, you can buy ammo but not weapons, equipment, or grenades there.
Integrating it with a repeater would only make more coding.
Integrating it with a repeater & medistation would take a dev 2 days to code the models so you could stand on it.