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It would help to know what year 3 series you're looking at. If it's an E36 ('92-'99), I may be able to help . . At 100k, the first thing that comes to mind is a radiator replacement. And definitely change the water pump if the seller cannot state or prove it's been changed recently. You'll want one with a metal or composite impeller, not the plastic type; not sure if you'd even find the plastic impeller versions these days anyway. Lastly here's a good read on what things to watch for:
oops, never mind, just noticed this is in the E46 section!