I think if you run and win the Special Condition race series at Costa di Amalfi (Normal), you'll be able to use the car I won and used here, the Subaru Impreza Rally Car 2001 Prototype. I knew I made a 118 A-Spec upset at Hong Kong using this machine. Since this rally car is a race car, you'll be able to modify any settings available for it, ranging from suspension to transmission. Also since it is a race car, you know its capabilities are much different from that of an average street car. I think the race requires Standard or Sports tires, so if you haven't done so already, get those tires. I recommend Sport tires, but if you're low on money, get the Standard Tires. They come free of any charge, so don't be afraid to get these Standard Tires if you're low on finances.
Now if you want a Boxer engine car with a little more get-up-and-go, I challenge you to run the "Race of Pleaides" Championship Series in Subaru's One-Make Race hall. If you win the championship, you get a more powerful, road-friendly race car (which can still be fitted with Dirt and Snow tires, per your request). Again, don't forget those Standard and/or Sport tires.
So if you want a car with a Boxer engine and low on money, go Subaru. If you want to splurge and you have the dough to back it up, pull into the Ruf showroom and get you a Ruf. I'd probably recommend the Ruf RGT.
On the track, just make sure you drive carefully and efficiently. I believe that a Boxer engine is completely horizontally-opposed, so a car can have an engine this small, yet provide such sufficient and effective performance. Just ask Subaru and Porsche. So why not enjoy the Boxer Engine for yourself? Win those races for GTPlanet, Sonu!