Dog lens help

I have a D40x and 55-200vr that I use to photograph my dogs outdoors. Very happy with the combination. Also have a 35 1.8 that I use indoors.

Just dropped the camera/telephoto in a lake. Have taken the front element off the lens and removed all the moisture, but don't know if the electronics survived. Will also take the camera to the shop and see if it is repairable.

I was thinking if necessary I would replace the D40x with a D3100. The extra stop of ISO performance would be nice.

So onto my questions:

1. The one thing my current combination can't do is keep up with dogs running towards me. Would the autofocus in a D3100 be able to predict the forward movement and compensate any better than a D40x (while using a 55-200vr)?

2. If not, I thought about buying a second hand pro body and old non-motor telephoto. This would no doubt solve the AF speed issue, but the ISO performance would likely be poor. Can anybody think of a body/lens combination that would be a good solution? I need it to be not overly heavy or expensive. A telephoto zoom or 180/200 prime would be fine. F5.6 is fine too as long as the body is decent-ish at high ISO.

Thanks!