These are all answers that can be found quite easily by searching
1. Dunno what you mean by fully adjustable. I'm sure you can do some level of adjustment with OEM. If you want to do full suspension tuning, you will need aftermarket parts.
2. The AT is actually better than the MT for the Z. It is supposedly maintenance-free. The AT transmission is definitely one of your biggest limiting factors when going FI.
3. Yes and no. You can turn off the TCS using the button on the dashboard. This does not deactivate other anti-slip features such as ABLS.
4. You're always going to be risking something when you buy a sports car used. Hopefully you can find one with one previous owner that garaged it and didn't drive it hard.