You should learn tracks in cars you want to drive. Whats the point in learning braking points for cars you aren't going to drive again.
As for how fast - it doesn't matter. If you learn the course with a slow car, and you intend on driving faster cars, then you'll be learning braking points which are too late and sometimes slower racing lines and vice versa. Driving a slower car doesn't make a track easier to learn, it just changes how you approach the track and which braking points you learn, etc.
Funnily enough, I'd say if you learn the track with an F2007 and then drive around the track with say, a Suzuki Swift, you're lines will be easier to learn than the other way around with going from slow car to fast car, but that means nothing - just learn a track with whatever car you plan on using.