1) What should you be aware when buying one?
Take your wallet, put it on the street in a parking space, light it on fire. This is what you will be doing to your wallet when you buy a 928. You can do it slowly or fast, but you are throwing money away. Be aware of that fact before you buy one.
2) What's the recommended minimum price for one?
Free. however if you want one that you can drive immediately expect to pay > 5000 . And it will still likely need work.
3) What's the fuel consumption (mpg/ l per 100 km)? (There's thousands of data out there but I want to know from your experience).
HAHAHAHAHAHAHA... OH GOD.. (COUGH) HAHAHAHAHAHAHAHAH.. (cough.. thats a lung) Hahahaha.. no wait.. your killing me.. no seriously call 911 im dying over here.. This is a V8 car .
4) How big is the road/engine noise while driving?
Stock, with all the weatherstripping, suspension and things like Motor mounts and exhaust in good working order, this was one of the quietest cars available in its day. There is some tire road noise (however this can increase or diminish depending on the tire brand)
Some questions you need to ask yourself.
1-Do you have a garage (with a concrete floor and space to have the car on 'jackstands' for days (weeks) at a time and the inclination to use said garage and jackstands regularly for the first year?
2-Do you have about 5000 (after purchase) to dedicate on restoring the car to its previous mechanical glory?
3-Do you have the patience to track down rare parts and follow complex repair procedures?
4-Do you want a car that is extremely fast, or do you want a 928?
You can get a mustang or camaro and it will blow the doors off a 928 (At least til about 120 mph or so) and do it much cheaper than doing the same with a 928.
Not trying to rain on your parade, I know from experience the 928 is like dating a supermodel, and then finding out she is bi-polar. Do you stay or do you go ? If you can date a bipolar supermodel you may be ready to own a 928.
Welcome to the bi-polar supermodel dating club.