The sloping top tubes of most all hybrids make it basically pointless to size by seat tube length. The effective top tube length will be much more important, especially given the fact that you intend to convert it to a drop bar.
I assume the bike you're considering is a used bike; otherwise, why wouldn't you just by a cross bike in the first place?
shwinn mountain bike,nakamura touring bike,and some others
answer are , yes ill be buying used (pawn shop around here are used bike shop literally)
and well even if id want a new one i would get a hybrid and make the conversion, a good hybrid is 500$ conversion is around 100$ online and getting some odds and ends at the coop. New cross bikes starts at 1200$ + (yeah market price sucks over here, the cheapest cross i found was 900$ for a 2007 norco ccx2 new never sold bike)
so yeah conversion indeed, ill try the bikes, i tought a couple meausrement would also help but it seems not as you say!