Replace Light Fixture with outlet
Light wiring is no different than outlet wiring in terms of capacity. The breaker should already be sized appropriately. 
Could be that the switch and/or home run (source) wires come into that box, and that two pairs go out to the other light boxes. Some tracing or testing would be necessary.
If the source comes into the box, you're golden. You can connect the new outlet to that and leave the rest of the circuit as-is. If not, You'd probably need to find a way to make changes elsewhere in the circuit to isolate that box. You probably can't bring additional cabling into that box as it's very full already.