I've had my 03 Track model since new, and I still love the car very much. What the 240Z was to the sports car world many years ago, the 350Z is to the sports car world of today. Nissan got so many things right on this car. Up until just recently, my car was untouched except for tires, brake fluid and brake pads. I would have a blast running with guys in Porsche Carreras of the same vintage at track days - they had more power and so would be ultimately quicker, but it was close enough to be fun. As far as upgrades:
- Tires...yes. I am a big fan of the newer Potenzas; they are grippy and have reasonable tire life. If you want the OMG factor, find a set of Hoosiers. In stock form, the 350Z can use them pretty well!
- Brakes...yes. I have always had Brembos (Track model) and they have never let me down. My 350Z friends with the small brakes have had more problems with wear and performance. The 350Z is a relatively heavy sports car, and so bigger brakes will certainly help. If the price of Brembos is a bit high, Stoptech makes a great kit for a bit less $$$. I believe all 350Z models have ABS, so with a set of big brakes, you will look like a hero going into the turn because these cars are incredibly stable in braking.
- LSD...yeah. A limited slip will certainly increase the fun factor, but for the money, not as big a difference as the first two. The VQ35 is not a power monster like a Vette, so you'll really see drastic improvement only in tight, lower-speed corners or corners with lots of elevation change. With that said, now that my LSD is getting weak from years of abuse with sticky tires, I miss it tremendously.
- Clutches/flywheels/etc...not really. I just installed an Exedy clutch and flywheel kit. It is definitely more grabby and DEFINITELY louder than stock. I did it mostly because I had everything apart anyway and bumped the power up a bit, otherwise I had no complaints with stock.
- Suspension upgrades will help, but probably makes sense only if you're on the track. I do find the stock suspension right at the limit of tolerable on long trips, so I think it will be relegated to shorter trips now that I have new dampers, springs and roll bars. I will probably swap the stock system back in when I decide to cross my California trip off the bucket list.
Welcome to the Z club! I hope it provides you with as much enjoyment as mine as given me!