Priorities of fluid changes

I bought this car used in December. The ABS light was on, and Trac off light was flashing. I drove it home, and the next day the check engine light came on. I have resolved the check engine light, the code was p0171. Expensive fix! Now the engine oil is clean. But the brake fluid is very dark! I have no idea when the coolant was last changed, nor do I know when the transmission fluid was changed last. I live on a fixed income, and I am currently broke from getting p0171 fixed. Once I get paid again, I can only afford to change the fluid out, only in either the brake fluid, or coolant, or transmission. Cannot afford to do all three with that check. I plan to have all of these fluids changed, as I have the money to do so, but it will take three months of checks to change them all. As I am on disability. Which fluid should be the first to be changed? The car has not overheated at all. But the brake fluid is super dark! Transmission fluid looks okay. Does not look new, but not bad like the brake fluid. Also transmission seems to shift okay. Sorry I am not mechanically inclined. I have to pay a shop to do these services. Thank you for your time, and any suggestions. PS: brakes do work, and there is still a decent amount of brake pad left.