IMO a collie whippet will be limited to what it can do, with exceptions of course as with all crosses. If your hearts set on collie blood then either a collie grey (if its half cross you want) or a collie/grey x whippet might be a good choice, bring down the size as you seem to want but still be quik enough to do everything a collie whippet can and more. I agree with what has been said, quarter collie is plenty, other three quarters can be tailored to what you want size wise. Personally though I would just get a pup from 2 lurchers doing what you want your pup to do...