Advice Needed on the Purchase of a Camera.

[Truckosaurus]

The Trackside forum is as good a place as any to ask this question in.

I'm looking for some basic advice about the purchase of a better camera than the one I have.

My current camera is a compact Olympus autofocus 'point and shoot' nice-and-easy job. It's good for snapshots but when I've tried taking photos at race meetings I'm either too far from the action for the 'zoom' function to get much action in the shot or else the autofocus focusses in on the fence or someone's head.

I was thinking about getting a 2nd-hand SLR but that's as far as my knowledge goes.

What size lens should I be looking to get to be able to get close to the action?

What other features should I be on the lookout for (other than autofocus)?

How interchangeable are different brands of camera and lens is there any one brand which is better served than others?

Thanks.