A subwoofer wont 'blow it up'
but vibration over time can crack panels, loosen glue, back out bolts, and basically just rattle the car apart, it's 15 years old to begin with. And it's a GTI, not a golf, so it's been driven hard it's entire life (regardless of what the previous owner says)
i guess it's ultimately up to you, but i would be very weary.
and also, i would wait until you have the car, and make sure you get all the kinks worked out before you plan on spending money on upgrades. And spend the $ for a new timing belt, cause if the belt goes, the engine is DONE. It's recommended you get the belt replaced when you buy a mkII regardless of how recently the previous owner says he had it done, unless he can show actual receipts for all the work.
i had a 99 16v with 49,000 original miles on it, and it ultimately cost me $1900 just to get it to good mechanical condition. Proper maintainence on those cars is $$$$ anymore, so people just plain don't do it, and 9/10 times, when people get rid of a mkII, there's a **** good reason. When they're running great, you'd swear you'll keep the car for the rest of your life, but when the shit hits the fan, it's like taco bell diarrhea, baby. One thing just leads to another to another, to another. And then rather than shelling out the cash, they just dump it on a new owner.
After dropping the $1900 to replace the timing belt, steering pump, a/c compressor, and a few other odds and ends, i just turned around and sold the car while it was in tip top shape, before something else went wrong, and bought another one with a 2.9 in it, just cause it was a newer, more reliable engine, with more available parts still being made (car manufacturers only have to make parts for 10 years from production...... so for a mkII with a factory engine, you pretty much have to rely on aftermarket and junkyard parts)