I have it and haven't spent a dime on DLC. Haven't came across a point yet where I've need any DLC.
Keep in mind I'm not one to sit and play games for 2-3-4 hours straight so I'm only about halfway through the game, I only game on weekends really late at night or maybe an hour or two during weeknights...
In all my years of gaming I've never walked into a store and purchased a game that I've wanted to play based on the resolution.
It's a great game with plenty of content to keep anyone occupied for hundreds of hours... however, the complaint about content lacking is valid because Forza 4 had way more in it. But there's no need to buy any of the DLC, unless it's a car that you just love and have to have in-game. It's also cool that they've been adding free tracks, especially ones as good and as important to North American motorsports as the ones they choose.
There are many hours worth of content on disk, I haven't spent any money on dlc and never once online or offline have I felt disadvantaged or restricted becasue of this. Also if you link your xbox account on the official website turn ten send you reward points. If you own any previous forzas like I did it gives you stacks of in game money so you can access the more expensive sports cars really early!