Hi Mate
You can get a 370gt sedan. I have a Dec 2008 build with the 7spd auto.
The back seats are very roomy, as it is more like the length of a 5 series, rather than a 3 series BMW (which is cramped).
We have a baby seat in the back, hooked up to the safety buckle thingy.
You are right, there isn't much that compares to these cars for the cost. I was going to get a 350gt, but the sale fell through, so I got the 370gt. They look the same (they look classy especially from a side veiw). Interior has minor differences. Obviously engine and gearbox are different, but I wouldnt expect performance to be much different between the 350 and 370. They are fast, but not 'really fast'.
Seriously, they are a great family sports sedan.