The base content will allow you to race in the Rookie series and that's about it. Some of the base content cars and tracks will allow you to race in higher race series, but only on certain weeks and they are few and far between. If you do take the plunge I suggest being very careful with car purchases. Lots of series don't very high participation and so don't go official and when they do then it's only the dedicated hardcore fans of the series that race.
Although the cost of entry means that you do get cleaner races, there are still plenty of idiots to ruin your race. You won't get some one deliberately driving the wrong way, but you'll encounter nearly all the other stupid things people do in public server racing games.
I've been a member since 2011 and there are a lot of things the service does well. But there is always more effort put into rolling out new content to sell rather than fixing issues that have been around for years.
I suggest searching for free month sub codes or 3 month discount subs before committing fully. Also bear in mind that without an active subscription you have no access to any content you have previously bought, not even to practice offline. You don't even have access to the forum without and active subscription.