I started with the VW Beetle 2.0. After all, it does have the MOST horsepower of your choices. No, it doesnt have turbo, but there are always N/A Tune Ups. On one of my saves(I have multiple memory cards), I am only on day 53 (game days, not real days) and only have about 3.5% Completed, but my Beetle does a top-speed of about 160mph (158 I think). Heres what I have done to it in such little time:
Sport Brakes
Muffler & Air Cleaner/Racing
NA Tune Up/ Stage 1
NA Tune Up/ Stage 2
Racing Chip
Clutch/ Single
Clutch/ Triple Plate
Flywheel/ Racing
Transmission/ Full-Racing
Limited-slip/ Full Customize Service
Lightweight/ Stage 1
Thats IT!!! Thats quite a BIG BANG for ure BUCK if ya ask me...