I've only had my GTV v6 a week so too soon to answer the fuel figures as I love the sound/feel of the engine so keep it in second a bit too long and let the revs roar.
Alfa Romeo GTV Buyer's Guide
has a useful buyers guide. With the V6 it should smoothly power up to around 7000rpmsbefore the safety cut out. So do that in 2nd to make sure the sensors aren't causing a power drop too early.
I expected a car with all round disc brakes to stop better, I'm sure my old focus with drums does a better job. So worth checking wear on the discs and factor into price.
When driving hard I find alot of heat comes into the cabin through the centre vents which is cured by shutting them on switching on the climate control. My climate controls comes on automatically, first I thought that was a fault but from the manual it's a setting which can be disabled.
My 5 foot nephew got in the back with 5`11 me in the front, his knees were either side of the front seat. Younger 4'5 ish niece was fine. Headroom was good in the back for a coupe.
Got 4 full supermarket bags in the boot along with my 2 tennis rackets, it's pretty deep, that's with the spare wheel too. just don't expect a large or medium suitcase.
Hopefully the long time owners can give you more details, by the way did I say I was loving my V6.