the credit transfer-thing i always thought was rather rediculous, as you get some rather good cars just from doing liscence tests, which can then be raced to get other cars. the only cars i bought in the beginning of the game were an AZ-1 for the K cup, some FF car for the FF race, and a damn truck.
but for me, the stats are:
*not started the s liscence tests, i wanted to get IA and then actually do races
*completed the beginner area
*first 10 missions
*some special condition races
*never use ASM, tcs at 1 if it's a high power car
*learning to use manual, though i'm lazy
*never bothered tuning cars, for several reasons: 1) Costs money 2) If you tune a car incredibly away from stock, then it's not really the car you started with. GT4 is a driving sim, not a "lets see how much power we can fit into a car and turn it into a monster" game. but i plan to do what i did in GT3 until my garage got too full: keep one version stock, or lightly modified (with things that would be easily possible in real life, suspension, chip, muffler, some slight engine/tranny work), and then one version that is heavily tuned. that way i have both to play with, and always remeber the roots of the car.
*my PS2 stopped reading GT4; that is, my PS2 broke enough and damaged GT4 (lense bumped up against the disc, but taht's what you get for buying one of the first PS2s on the market and then having idiot roommates).

And I had quite a few finals to deal with, not to mention preparation for moving to Japan, where I done am now.
So a few days before I left, i bought a PSTwo (buying PST for primarily travel reasons, friend will repair and use PS2), checked that it would play GT4 after my PS2 damaged it, and finally got a place and a tv. But it's been a month to the day that I haven't played GT4, and i plan to finally play some tonight.
woo! why am i here then? i could be playing gt4!