With the Alpine VGT road car I always win with 1-1.5 minutes lead, depending on the weather. Its super fuel efficient, if you do it right you can easily do 4 laps stints if you do a little short-shifting and coasting and it does 305 km/h on the straights de-tuned to 699PP.
It costs 1 million but its worth every penny, it can be used in so many races and it always absolutely DESTROYS the opponents. Its totally unfair.
Another really good car is the 68' Alpine A220 racecar. It also has an amazing fuel efficiency allowing you to do 4 laps, its got gerat top speed, handles very well. Just make sure you max out its downforce so it stays put even in the rain. I ususally win with at least 1 minute lead with it.
But that's not fun, that's just pure grind for the money. Use your tuned cars, if they can match the top speeds of your opponent Group 4 cars its gonna be fun and winnable.
Just one word of advice - stay away from cars with super thirsty engines, if you have to pit every 2 laps you can still win but pitting so many times gets suuuuper annoying. Cars that are really thirsty: Aston Martin One '11, pretty much all of the US muscle cars with V8's no matter the date of production, the Japanese cars from the 90's, the V10 and V12 engines in the Italian supercars (Lamborghini, Ferrari). New production cars with smaller engines have very good fuel mileage, even tuned to Group 4 spec.