Depends, I tend to buy what I know and never buy anything without a test drive anyway.
Car purchases I remember well:
1) First car. Decided whilst learning I wanted a Mk3 Escort, nothing else would do. It HAD to be a Mk3. Month or so later, passed test, bought Freeads that Wednesday, bought car that night.
2) Drove past a Sierra at the roadside at night. Decided I wanted it. Went back following day, test drove and purchased.
3) Someone at Uni asked me to find an estate car with an MOT. Rang up a bloke that night who I knew had a Sierra. Picked it up the next day.
4) Got a new job, realised car was shafted. Decided I needed something cheap and needed it NOW. 10 minutes later a mate was on the phone sorting out my Mondeo. I collected it an hour later
5) Decided I wanted another 205 GTI. Didn't need to research, had em before. Phoned a random ad an purchased all on the same day.
I don't really think stuff through much. If somebody offered me something interesting when I had the money to buy it I'd probably just do it on the spot.