Generally, lenses are not interchangeable between manufacturers (in this case Nikon and Canon). So if you buy the D50 and get Sigma lenses, you're going to have Sigma Lenses with a Nikon mount. Lenses are more an investment than the camera body. The D50 camera sensor is fine (if long in the tooth). For 100 bucks starting out, depending on the quality of the camera (any dust on the sensor, or in the body - you can tell by looking through the view finder or taking a picture of a grey piece of paper and looking for artifacts, also what are the shutter actuations - for 100 bucks I'd say if it had half of its rated shutter life, I'd still buy it) that would be fine, although you will be "limited" to 2 frames per second action shooting. Ask yourself though, if your need for frames per second overwhelms your need for a good deal on a camera. You'd have 700 bucks to play with, and be able to afford a decent walk around lens (that Sigma 24-60), and a nice telephoto to satisfy your action needs, and have some change left over to grab a Sigma 105 f/2.8 for macro and portrait shots. When all is said and done, and you're ready to upgrade to a D300 (they will drop in price, right now people are regularly unloading them for 1200 bucks) you'll have a nice stash of lenses that will be able to compliment the D300's Hi-ISO and fps capabilities.