Just the lens will cost the entire $700. I like the pentax bodies for their quality and weather resistance.
For sports your looking for a zoom. Something like a 60-250mm would be ideal. 60mm allows you to get the entire field, and 250mm zooms in on the kids. 135mm isn't really much zoom and I'd prefer more like 250mm-300mm. When you get into 400-500mm it's a heavy lens requiring a tripod and not great for hand photos.
For the night aspect you want a fast lens with a large aperture. Cheaper lenses will be something like F3.5-6.5 for example. So when zoomed in at 250mm it's equivalent to F6.5. A more expensive zoom will have a fixed aperture like F4 across the entire range. F4 vs 6.5 makes a big difference at night and lets in way more light.
Pentax has a 60-250mm F4 that is weather resistant for about $1200. Canon has a 70-200mm F4 for $600. There is plenty available. A $1200 lens will have better image stabilization and autofocus.
Tamron makes a 70-200mm F2.8 in Nikon, Canon, Sony, and Pentax mounts for $750. The F2.8 is awesome and would give you an intro into night photography without breaking the bank. But buyer beware, autofocus and tracking can be a problem with lower prices optics.
Check out bhphotovideo.com to see what's available and pricing.