Yes the general consensus seems to be about 17mpg for the V8. To put it in context that's about the same as a Brera V6. The M3 has a 60 litre tank so a bit smaller than we're used to (Brera/Spider/159/GTV were all 70 litres), so you're looking at 50 litres to the amber light in an M3, as opposed to 60 litres in our Alfas.
MPG doesn't bother me too much as I commute by motorbike and bicycle, and the wife's diesel is the family car. This car would be strictly for me for fun (and for taking my little boy out for fun, he loves cars). That's why i think it's time for a bigger engine - we don't rely on this car as main transport - though we could all fit in it if we needed too.
The servicing costs can be high, but the service schedule goes like this (main dealer prices):
Oil service ~ £150
Inspection service 1 ~£600
Oil Service ~£150
Inspection service 2 ~ £900
Repeat from beginning.
So the trick is to buy a car that has just had it's Inspection 2 service, plus new tyres and brakes
That's the ideal scenario anyway. However, the onboard computer tells you how much life is left in each brake in 1000s of miles, so the brake situation will be obvious as soon as you start looking through the iDrive system (the driver's computer interface). So really what you want is a car that's just had Inspection 2 done if possible.
However, it's hard to predict when the next service is due from looking at Autotrader alone, as the service schedule is determined by the on-board computer looking at the oil degradation sensor (as far as I can tell). Just have to ring up each seller and see what's been done. Sadly many weren't picking up over the bank holiday weekend.
Good luck with the fuel costs. On nearly all of the reviews concerns were centred on fuel consumption, and to a lesser extent service costs. The 'standard' 30,000 mile service cost for one owner came to £1,000. But the return for one reviewer of just 160 miles from a full tank of fuel is eye-watering. They were 'filling up' every day. Incredible.