yeah, the Fiat 500 is really hilarious! With its top speed of 120kmh or so its so easy to drive, that you think its a super race car - until you realize how slow it is.
Assetto Corsa is not the worst, but also not the best racing sim. There are a lot of different sims around, and a lot are very specialized, like Truck racing, Nascar, F1, Dakar Rally, WRC and whatever. All good if you are a fan of one of such series.
But as a allrounder with maximum realism, plus a maximum of different cars and tracks, I do recommend Project Cars 2. Its not new, so a bit weak on the graphics side, but really good in the physics of car and track simulation. Especially for the use of a steering wheel, which is basically a must for this sim.
That being said... a racing purist might claim, that the only race track you need ist the Nürburgring Nordschleife. If you mastered that, you arrived at the Olymp of race driving. :)