I don't know what 'suitable' would be for a single race in Arcade mode but, we know that a Ferrari 458 costs 283.000 credits. And it seems that single race allows you to choose a car which you currently are not "equipped" with: he has an Atenza, race is with a M6 Walkenhorst. "Suitable", to me, depends on the planned progression. Perhaps earning too much, too quickly (one lap Nurb it seems?), may break it. The price, and the career prize cars under wraps. XP/Lv may be used as gates for certain events or even features. Or just be another "gauge" of progression/time spent.