So, the engine itself is actually quite impressive when it's running properly.
Whenever possible, you should definitely buy the car with a used car warranty. Even small repairs can cost a lot of money, not to mention the mass airflow sensors, which are supposedly now supposed to last forever...
The Passat V6 TDI 4-Motion is essentially a Quattro, meaning the technology largely originates from Audi rather than Volkswagen.
Regarding the weight: I own a 1999 Audi A4 12/98 (VIN <200,001), AFB V6 Quattro with full equipment (including tow hitch), and according to the TÜV scale in October 2003, it weighed approximately 1710 kg with a full tank.
Speeding is no problem; it also handles corners really well, but the weight makes braking quite difficult.
To address this, I installed larger brake discs on the front axle (312mm instead of 288mm), which made it much more tolerable; also, the wear is reduced.
Regarding tuning: I got a chip from Noth**le back then and I'm very happy with it, except for the soot coming out of the exhaust. It runs very smoothly and easily. The advertised power output was 180 hp and 370 Nm, but the car actually exceeds those figures (measured). However, I can always swap the chip back to the original one; which is what I always do before visiting a workshop.
Regarding fuel consumption: I rarely manage to achieve 8 liters per 100 kilometers, regardless of whether the car has a performance chip or not; closer to 9 liters, or even 9.5 liters, is more typical, with an 80/20 ratio of highway to city driving.
However, I've heard that the new engines are significantly more fuel-efficient.

Stefan.