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??