The question what ND graduated filter to get first is quite hard to answer. See: What ND Grad to buy first?
I had the idea to look at the images I already have taken in situations where I would liked to have a grad filter with me by applying a grad filter in Lightroom and then find the right grad filter for me by looking at the settings of the filter applied in LR.
Wether this works or not splits in two questions:
How accurate are the exposure values in Lightroom and on the actual graduated filters? Can I expect a somehow equivalent result from a 1-stop filter in LR and a 1-stop grad filter?
How can the size (or softness) of the grad be determined from a filter in LR? This should be depending on the actual focal length but Im pretty sure that some things can actually said about that. I think we dont need to be too precise here.