When I wanted more luxury 9 years ago I too moved from Nissan to an Infiniti. I went from a 99 Maxima to a 01 I30t. 14 months later feeling the need for more sport I traded for an 03 G35 coupe Journey which I put 45k on before trading for an 08 G37 coupe Journey. After some 30k I traded for an 10 FX35 which is my current vehicle.
Since you state that you intend to spend 30k +- The problems of the first few model years are not relevant(windows-seat assy-head units). For that kind of money you should be able to get into a G37. The G37 was introduced in 08 and include the larger engine with 330hp and a redesigned interior far superior to the G35. Starting in 09 the 7 speed auto transmission became standard. If you like manual everything is a 6 speed
Now for your talking points:
(1) I would not expect the ride to be vastly superior to your 350z. Remember we are a really only talking about an esenntially stretched 350Z. You are still in a short wheel based sport coupe with low profile tires. It is heavier and not so tightly sprung as a 350Z so it will ride better but not that much. I suspect the amenities inside will make up some but that a decision you will have to make.
(2) The G37 is very tecnically adanced. Bluetooth and Ipod connectivity come with the premium package. Navigation is a separate package. I suspect without any proof that almost all G37 came with the premium package. Shouldn't be that hard to find.
(3) The stereo in the Infiniti coupe is to be polite substandard. For a car that has pretensions of competing with other luxury brands the Bose premium system just doesn't cut it(some less polite than me will tell you it sucks). The low end is mostly non-existent to poor and the rest is just mediocre. For a great stereo you will have to go aftermarket.
(4) Even though I had numerous problems with the 03 coupe it was drive train dead reliable. None of the problems I had ever put the 03 out of service in almost 5 yrs. The 08 G37 was 100% reliable for almost 3 yrs before I traded. I 've been driving for almost 40 years and these cars were the most fun and distinctive cars I ever owned.
To conclude The G coupe is a great car to own and drive as long as you can live with its limitations. Yes it has a backseat but only for those under the age of six. The trunk is only barely useable and the G is absolutely the worst cars I have ever owned to drive in the snow. Ultimately it was these limitations that after 8 yrs in G coupe led me to buy the FX. Since you have been in a Z for 6 yrs you surely understand these limitations. Sorry to be so long winded. Good Luck!
