Just to answer your questions in order; yes, they should be good for rabbits and ferreting; yes, they could be ok on the lamp for rabbits; and I would say a 3/4 should definatly be faster than a 1st cross ! Personaly, I would rather go for a Beddy/Grey as an allrounder than a Beddy/Whippet, as I find them to small to be good allrounders, but many people will disagree with me on that one !!
Cheers. thats true