I wouldn't want to go far in the back of the mustang, it's the headroom more than anything. The noise is almost non existent unless you go aftermarket. I get around 20MPG but it's only a weekend car. But have seen 35 on a motorway.
The prices of brand new Mustangs are still rising. Not sure what that does to the second hand market but I think since I ordered it has increased by at least a grand, with an increase in the last couple of weeks.
Edit - there is still a 7 month waiting list if you order new, but the head of Ford UK said on the forum that there are 19 unsold cars around the country.