If you DO go Nikon, Keep in mind that the Nikon D40, D40x, and D60 will NOT support autofocusing of your old lenses, but they will still work.
D50, D70, D80, D90,
D100, D200, D300, D700,
D1h, D1x, D2h, D2x, D3 will all support autofocus on older lenses.
The bolded cameras should be within your budget used but are discontinued.
Personally I would try to find a new D80 (freshly discontinued), with an 18-200mm VR lens. Good all around walk around lens which should suffice for sports shots so long as they're outdoors.
That should fit within your budget.
Or if that is too much.
Buy this:
http://www.bhphotovideo.com/c/product/541241-REG/Nikon_9483_D80_SLR_Digital_Camera.htmland then get the 55-200 VR lens for $225, you have the same focal length, 18-200 with great VR use. The only thing this won't be great for is indoor super low light stuff. For that you likely have an old lens like the 50mm 1.8 to use.