A 2.5liter, if I remember that's 168hp and 185ft-lbs, should be in the middle of ITS. You'll have to request it be classed as only the 2.8 liter is classed at this point.

Do you have a hard top? That'll add $1k - $1.5k, and you'll need it otherwise top speed and fuel mileage will suffer. Got a welder, can you align and corner balance it? Parts wise maybe $3k to $4k. The coil over kit with springs and camber plates is $2300 from TC. With the 2.5 it should do pretty well on fuel. I've got a friend that's done well in E3 with his JP Z3. He does have to drive smooth, but he's done a 3hr enduring on one stop even with the 14gallon tank.