i have been researching about the 30k maintenance checkups. so far ive done everything that is necessary except the fuel filter and coolant flush. i was intending on doing the coolant flush myself and adding the 50/50 coolant mixture myself, but my friend who is a mechanic highly advised me that a "pump" was necessary to take out all the coolant. is this true? is it worth to save the time and effort of a coolant flush and take it to the dealership?

my next question is about spark plugs. it seems as though its nothing but mere opinions from all my searching..but how much of an overkill is it to replace spark plugs at 30k?

Spark plugs in the WRX are platinum and aren't required to be replaced until 60K. You certainly have the option to replace them at 30K but they'll probably look like new when they come out.
As for the coolant it isn't necessary to do a complete flush for the regular maintenance, it's quite sufficient to drain the radiator normally through the drain plug. This will drain out almost all the coolant in the engine. Just be careful when refilling the system, the 02s with just the single cap on the engine and no radiator cap are prone to developing trapped air pockets when filling. This can result in overheating unless the air is purged. Follow the procedure in the manual for filling and "burping" the system.

IMO -- time and effort wise -- doing the coolant yourself isn't worth it. But at the same time, knowing it has actually been done and done correctly (by your own self) gives the greatest satisfaction and peace of mind.

Its easy to flush all the old stuff out. Turn the heater control to the full hot position. Disconnect one (doesn't matter which) of the heater hoses under the hood and put a garden hose with a nozzel in the heater hose. With the radiator drain open, and the radiator cap off, turn the garden hose on at a low pressure. Randomly block open holes (radiator drain, cap, open heater hose line) until all the water coming out the remaining open holes is clear. Easy and effective.

Did you also flush/replace the brake fluid?

nope. havent flushed/replaced the brake fluid either. i was researching on that and the job itself seems somewhat complicated. thanks for the idea of the coolant flush.