1. The majority of Z's are pretty reliable, so as long as you find one that has been well taken care of, you shouldn't find any major issues. Maintenance stuff like brake's and tires are a bit more than other "normal" cars, but it comes with the territory. Be prepared for $90 oil changes unless you plan on doing it yourself. 15k miles isn't really all that much, neither is 30k miles. The Z shouldn't need any major maintance work until 80k-100k miles.
2. It will only void warranty for the doors and anything associated, but I say HELZ NO to lambo doors! Just don't
Please don't!
3. Installing the OEM nav is going to cost a lot of $$$. You can go with an aftermarket nav, but the screen isn't going to be in the cubby. Spend the extra for the OEM nav if you really want it, otherwise you might regret it later.
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Oil change: ~$60 for DIY-$90 @ the dealer (Nissan Ester)
Brake pads:~$120 about every 35k miles, give or take + install
Tires:~$1,000 for a set depending on the brand. Will last about 15-20k miles.