Pulldowns could be a very frustrating thing if your intention is to have them lead to pullups. There is not a very good carryover. I've seen this effect myself a couple months ago. I had a freind over who is a naturally large muscled and strong guy...he couldn't really do any full rom pullups. Although he did a few partial. But when he got on my lat pulldown I couldn't seem to find enough plates for him
I agree with Ross that assisted pullups would be a better bet.