Am I ready to move online?

All of them lol

You can pretty much get away with just having the basic cars, especially if it is just a freeburn room. But, you might want to play offline and unlock some cars just so you get to know the city a bit better before venturing online.

If you are going to be doing online challenges it will help if you roughly know where you are going, and how to do barrel rolls and flat spins.

I would avoid race rooms until you have some boost 10 cars unlocked. IMO the offline game is pretty easy so it shouldn't take you long to get some competitive cars!

As ZombieTron said  if you're just freeburning and doing challenges you can hold your own using the earlier vehicles. The main element is learning the map and being able to do barrel rolls, flat spins and power parking (it's amazing how many people can't figure this out).

If you want to delve into the murky waters of ranked racing you will need some of the level 10 boost vehicles, be warned though a lot of people are using the Nighthawk for shorter routes and the Extreme Hotrod for the longer ones. Both of these can be beaten with original cars depending on driver skill.

be warned though a lot of people are using the Nighthawk for shorter routes and the Extreme Hotrod for the longer ones. Both of these can be beaten with original cars depending on driver skill.

Well there are a couple of high ranked players on live that will disagree with that.

I've been tanked a few times around paradise with extreme hot rod.  It's shouldn't be possible:(

All depends on the driver!

Its amazing how many people online think that having the faster car should make them win.  With how many different routes and shortcuts there are it's not always about speed.  I've seen no end of people online go whizzing past their turn beacuse their car can only go fast in straight lines.

Like others have said if you want to just do some freeburning or challenges then pretty much any car will do you but for racing I would recommend just getting to learn the city and find a (quickish) car that you are comfortable with.

 I'm sure that some of the people here at Burnoutaholics could guide you through your first days online.  We are a community, after all.