Brake fluid should be done every second or third year depending on the humidity of the air in your area. This is regardless of mileage/age of the car. 2-3 years tops.

Coolant is about the same, though many coolants are called 'extended life' now and say they will last for 5 years. I think 3-5 years is probably what's right, and it's up to you really. If it doesn't say 'extended life' on it I would change it every second or third year like brake fluid. Again, this is an 'actual time' interval and doesn't have anything to do with mileage or age of the car. My own research has brought me to conclude that the Zerex G05 is the best commonly-found replacement for Volvo's OEM fluid. I bought mine at CarQuest. (sidenote: I think if DEX-COOL was a decent coolant it would be fine too as it's designed for GM's all-aluminum engines, but there have been a number of complaints about DEX-COOL so I'm staying away from it. I used it for about a year though.)

In order to answer your questions on the thermostat/water pump thing you'll have to tell us which engine you have. I know on the 5 cylinder engine that went into service with the 850s the thermostat change has no bearing on the water pump. The water pump is accessed by removing all the timing belt stuff, so it's usually considered at the same time as that job.

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1998 V70 AWD->FWD Turbo 200k+