I have bought all the cars in the game, and will buy the 3 that will come tomorrow (assuming they are available at Hagerthy right away).
For now, I'm buying duplicates of a lot of cars that I want. Most of the duplicates are going to be road cars as those are the ones that can be upgraded with "permanent" tuning in the form of wide chassis and engine tunes.
Some cars, namely the already powerful hypercars and supercars, I will buy triplicates of them to have them like so:
-1 stock (with normal street parts but no permanent upgrades)
-1 fully tuned
-1 top speed version (basically, all permanent tuning included aside from the wide bodykits and other parts).
And I may even have to buy a quadruplicate of some cars because of the Engine Swaps and Special Car Parts. Mazda RX-7 is going to be one of them. ... ...... Hopefully by 2030 I will have the 787B engine swap.
Only problem I have is with some invitation cars where I forgot to buy an extra of them. Like having 3 Enzos and 3 Carrera GTs and 3 Veyrons and I "only" have 2 of them. Gonna have to rely on luck to get the invitations again.
So, it's going to take a while to get all this. So I'll just keep grinding by doing 2 or 3 Tokyo races and earning 2 million per day.
And before you ask, yes, I have 2 road version F1s, and will get a 3rd. almost 60 million on a single car.