Well if I were you, just stack all scrolls you have obtained and wait for Lotto Mouse event or any event that gives you reward in completing the daily that involves using scrolls. That way your scrolls are worth spending at the same you might get powerful hunters.
If you insist on buying them, probably the 3-4☆ request. It's got a chance of a 4☆, plus you can sell the 3☆ you get if you don't need them. Personally, I don't think you should buy, and just save up (-: