I got tired of just buying ECU tuning and catalytic converters and blew all of my saving on a Lamborghini Gallardo which I proceeded to max out.  Next I think I'm going to try and fully soup up a GT-R SpecV even though I probably should find an FR car instead.  I always end up favoring Nissan cars the most though.
 
What car finally pushed you guys over the edge?  Any cars I should think about starting work on?

Probably the Civic I started with. I've fully upgraded probably around 100 cars by this point, some I only drove for like one or two laps :/ In the later stages of the game, especially when you get the X1, money ain't really an issue, so you can buy and upgrade pretty much anything you feel like!

Stock GT-R

A blew a lot of money on the Skyline M.N.P edition based purely because that was my ultimate street tuner car from previous games. I also blew all the money I had at the time on the Carbon R in the used car lot since that is like the ultimate Track tuner version of the Skyline, until I realized I was being stupid since the Carbon R is a reward car later on down the line. I actually didn't put that much money into my first car since I did a bunch of license test and won more cars to actually get attached to my MR-2.  
 
Most efficient way to make money is the American Championship where for 5 laps around Indy you get 117k but you have to use an american car. But I would focus on leveling up over money since the new car lot actually opens up and has more cars to buy the higher level you are and I cannot tell you how many times I saw a car I wanted in the used car lot, but couldn't buy it not because of money but because I wasn't high enough level. 

A Mustang Mach 1, I love that car.

These are the ones that I went all-out on and bought the "Engine" mods to fully tune. In order of purchase:
 

1. Mercedes-Benz C 63 AMG '08
2. Audi R8 5.2 FSI quattro '09
3. Subaru IMPREZA WRX STI '07
4. Toyota PRIUS G '09
5. Subaru IMPREZA Sedan WRX STI '10
6. Chevrolet Corvette ZR1 (C6) RM '09
7. Subaru IMPREZA WRC 2008
8. Nissan GT-R SpecV '09
9. Subaru CUSCO DUNLOP SUBARU IMPREZA '08
10. Subaru IMPREZA Sedan WRX STI spec C Type RA RM '05
11. Audi RS 6 Avant '08
12. McLaren MP4-12C '10
13. McLaren F1 '94
14. Lotus Elise 111R RM '04

WRX STI.

Audi R8 5.2 Quattro. With everything, still working on tunings.
 
Fuck me, it's a beast.

Prius. I have no idea why I bothered.

" @Binman88: Haha, I'm quite a ways away from that unfortunately.  Was there any point when your income really took off?  I'm level 15 right now and it seems like running the international speedway in the supercup festival for 20k every 2.5 minutes is the most efficient way for me to grind it out. "

" Prius. I have no idea why I bothered. "