Whatever online mode you play, you should have access to every car in your garage...
If you are trying to take part in an event for which you do not own a car, you may have to use a "rental".
As for difficulty, just go by preference; find the experience that's right for you... Don't worry about XP / money, just have fun; you'll have more than enough of both before you know it!
Sometimes though, you may just have yourself a "weak" car that is rated higher than it should be and that could give you a hard time @ unbeatable AI... But this is very rare, especially if you load 5 star tunes from the cloud (which, unless you really know what you're doing, you should ALWAYS do!)