With a budget of $12-15k I'd be looking more towards an end of cycle 2007 or 2008 350Z than an early model 370Z.
You get the best version of the VQ that ever went into a production 350Z, no oil consumption, no transmission issues, no window motors issues, and a far better interior than the preceeding 350s. Additionally, there's a lot to choose from. That's a good budget to get a decent Z33. You can avoid the RevUp VQ if you add the models from 2005 to the mix if you avoid the 35th Anniversary Edition and the Track Edition, but you'd miss out on the VQ35HR from the last two (full) production years of the 350Z.
Now go run a search on cars.com, or another similar site, for a 370Z with a budget of $15k. You don't have a lot of options, I imagine most are probably early models that have a pile of miles on them...and probably not a lot of TLC.
In the end it's your money to spend, I'm just letting you know what I'd be looking to buy if I were working within your budget...
...tires are also cheaper for the 350Z.
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