Actually the last 8V GTIs didnt have rear discs. I dont know when they started, but by 92 if you have an 8V GTI with discs in the rear, you have something special.
Unless the previous owner was racing the car, chances are the diff is not bolted, its still riveted. Actually if I was asking a seller if he bolted the diff and he said yes, I would figure he was A) lying and had no idea what I was talking about, B)knew what I was talking about and lying, or C) beat the living snot out of the car while he owned it.
Fact of the matter is, if you drive the car resonably you will never have a problem with the rivets, and most of the time you will just bust the spider gears anyhow. Rivets or bolts on the diff I wouldnt waste my time asking.

You do want to look for rust. usual problem areas are at the corners at the base of the windsheild, under the rear 1/4 windows, and the body seam at the rear under the taillights. Also, the floor pans have a nasty habit of rusting if the car was allowed to leak water for a long period of time, but due to the fact that the underside is undercoated you dont even see it by looking under the car. Only way to tell is by pulling up the carpet. Also check the shock towers in the front for rusting especially around back. Cars from the north get salt slung up in there and wreaks havoc.

Why would it need a clutch/brake pedal assembly??

 

I understand. But, unless hes BUYING an autox car, its probably a waste of his breath to ask.
As to breaking diffs with stock power, much of it is in how its driven. If the car is abused, the diffs will break. If the car is driven reasonably, they will last a reasonable amount of time.
Back in the day I was busting trannies right and left. I had just about singlehandedly cleaned out Cophers in Tampa AND Clearwater, as well as Ace of their 5 speed boxes. At that time I had an 8V rocco with a later model hydraulic head, header, exhaust, and a filter. I also had a puck clutch and routinely did clutch dump burnouts and powershifted the car.
Trannies would last me a week or two.
Fast forward to a few years ago, I had a stock 2Y box in my car for years, no problems. That was with a 1.8L 16V a 2.0L 16V, then a bad attempt at a 16V turbo and finally a very successful 8V turbo setup. The car would boil the tires by ROLLING on the throttle in 2nd gear. Never had a tranny issue until finally when 5th ate itself from a lack of fluid on a trip to Tampa.
I honestly think that what makes a huge difference is the way the car is driven and the cars maintanence. I never have a problem with hydraulic mounts. Then again, when my stock mount was wore out I bought a new one instead of a junkyard mount. And I put in all the bolts, including the one that goes through the starter.
I also have outgrown the practice of revving the car up and popping the clutch on every corner. That is basically beating on it. A 200hp turbo car will do a much more spectacular burnout and be putting more overall power down with a rev and moderate application of the clutch than a 110hp car will by having the engine revved to 6K and the clutch dumped.
But the shockload on the driveline parts will be ALOT higher in the second scenario, resulting in more broken parts.
Im not saying a bolt kit is a bad idea, but most of the people I see saying its a must are just driving retarded. If most of these people would quite driving like they are 17, their trannies would last alot longer.