I say drop the 15k into a TT, get all the ALL the 'safe mods', and hoard cash. As you mentioned earlier, it would be tragic to wreck the R8 or GT3 on the track. Not so much with the Z. If you feel you want to upgrade after a few years of hoarding cash, then upgrade (to the GT3!!!!).
While not in the same financial situation as you, I went with the Z over the Exige S because I would be devastated if I wrecked it at the track, and I can hoard money and wait for some nice investments or pick up that second home somewhere. Heck, my wife is from the Bahamas; maybe I will build a house there to vacation at in the summer.
Do something like that; once established there, hook up the GT3!
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