Believe it or not, you can sweep the Professional/FF Challenge races with the VITZ 1.5 RS(J). The toughest competitors the AI chooses are the Spoon Civic and New Beetle Cup Car (I've never had to run against the Xsara Rally Car).
Here are the upgrades I purchased for the VITZ...
Suspension/Sports (-16000)
Sports Brakes (-4600)
Brake Balance Controller (-10300)
Muffler & Air Cleaner/Racing (-4600)
Port Polish (-5250)
Full-engine Balancing (-11000)
Racing Chip (-1250)
Clutch/Triple Plate (-4600)
Flywheel/Racing (-1000)
Transmission/Full-racing (-10200)
Turbo Kit/Stage 2 (-13000)
Racing Tires/Super-Slick (-10250)
Lightweight/Stage 1 (-1100)
Lightweight/Stage 2 (-5300)
Lightweight/Stage 3 (-21000)
= 119450 cr spent
Here are the times I've recorded with the VITZ and the Xsara (the Xsara had an oil change and Stage 2 Turbo)..
10 Laps @ Deep Forest Raceway: V = 14.20.907; X = 14.19.806
10 Laps @ Rome Circuit: V = 16.25.038; X = 16.15.761
10 Laps @ Cote d'Azur: V = 18.37.395; X = 18.34.355
Of course, there are going to be more powerful vehicles qualified to race this set of races... But since I felt it necessary to upgrade the VITZ for the VITZ Races, I may as well use it where I can. This also allowed me to keep the Xsara in mint condition, as well as save money for any upgrades I would've spent on it...
Anyway, the VITZ might not be the "Best FF Car", but it's probably a lot more useful (in this game) than others might think.
Have a great drive, everyone!
Hazzard