So far this has been my trick with all cars, buy and go into the upgrade shop (not quick upgrade) Now decide what you want the car for, if its for non-class restricted racing, ie you want maximum power and performance, then go to the far right and look at engine swaps or aspiration changes and buy as necessary, these upgrades affect the other available upgrade trees and so its cheaper to do this first and then do the other part swaps.
If your car is for class restricted races DO NOT change the aspiration or engine or drivetrain as you will likely increase your performance index a large amount without gaining much immediate performance.
Now the general upgrade pattern following those mentioned above goes like this-
Brakes and tyres, max them out super wide slicks on aftermarket rims, changing the wheel dimensions only seems to affect the car negativley so i leave it out.
Weight reduction - maximum weight reduction recommended
Aero - Add both front and rear adjustable spoilers
Engine mods- Apply as many of these as you can until you are around 30 point off your car going up a class(obviuously if you are modding all the way just buy whatever you can) if your car is turbo focus on turbo upgrades
Transmission - use whats left of your Pi to improve shift time
Seems pretty obvious and im sorry if anyone feels like im teaching them to suck eggs or whatever but this has left every car ive driven miles ahead of the competition in power and performance ith little need to fine tune.
Good tip for cash, fully upgrade an NSX GT-R to R3 class and hire a driver to do the R3 endurance race over and over, this will net you 85k per session at 63 mins a throw, ive got up to about 1.5m now without much driving, you need to be level 20 to unlock the endurance events. These events are great for levelling up too, the A class endurance at suzuka takes around an hour and will net you over 90k XP which will put you up a bunch of levels until you are level 30 or so and the R class events will net you over 100k exp
*note XP given is equal to the default cash reward for the track
Again sorry if the big forza heads find this post a bore but im new to the franchise and these are my tips so far that are working very well