I cant help with the Mustang, looks like marzman is the man for that, but they do look amazing.
Regarding the 370Z, I have had mine for nearly a year and I have loved and am loving every minute of it. The sound, the reliability, the looks and the performance are all exceptional. I use mine every day to and from work and I love it. I have owned some pretty amazing cars in my time and I thought that "proper" car ownership was over when I bought my Vauxhall Corsa and then a Ford Fiesta. Then like you, I suddenly thought, Sod it, I am not enjoying this. I am 57, yes mid-life crisis it may be but I have always loved the Z, I had a 260Z back in the 80's and always loved the 350Z and then the 370Z. Now I own a 2012 370Z GT Edition with just 6000 miles on the clock It is mint and I totally love it. Cant see this one going for quite some time yet.
Regarding the Maserati, DONT DO IT!!! I worked for the local Maserati/Ferrari dealership for 14 years and saw the Maserati progress from the 3200 to the present. They have come a long way and they are a lot better. There is no better start up roar than the Gran Turismo and they do drive very well but and as you have said the repair/servicing costs are astronomical. I was on 1st name terms with a lot of customers purely because there car was always in for something or other. Even if you can find a cheaper labour rate the parts are very very expensive and things like brakes and steering don't last very long.
In my opinion and it is only my opinion go for the Z