Well, I went with the Fairlady Z R3 car. Has a huge braking and handling rating, plus it's acceleration and launch is really good too. It lacks a bit of top speed, but it can be compensated by gear ratios and the unlocked racing engine upgrade ;) Also it was only 200K credits.
I'm on the first R3 championship race too, and having a bit of trouble controlling the car at first too, this would do so much better had I got a racing wheel ._. I really hope that somehow, someday MS allows G25 compatibility, through an update or something.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"