I buy cars multiple times due to the payouts. After that I tune them with all parts (which don´t change weight or power) and tweak the settings.
After that I look at the pp the car has and then it get´s his optimising. For example:
The M3 from the NCD has 491pp after oil-change
=> I made a 500pp Car out of it (my favourite Class for online)reached with small power upgrades (or small weight reductions can´t remember now).
At the beginning I maxx tuned my cars but now I just do the above thing because maxxing "kills" the driveability of some cars (maximum now is the small power upgrades and weight reduction 1 and the window/hood)
I have a phobia of tuning cars as well, and for good reason:
I test most of the cars I own on my course-maker-made test track for the fastest lap time possible, a la Top Gear. In order to get results as accurate to the real cars as possible, I obviously need to keep the cars stock.
Yes, almost all cars I like to keep stock (unless they are pre-tuned of course like the HPA TT etc) Only thing I tend to sometimes change are the tyres.
I keep all of my supercars stock, all of my classic cars stock, and all of my rare cars stock. But I do love modifying Civics and NSXs and Evos and stuff like that. Although I do have a few modified muscle cars and one or two modified super cars.