the general rule with timing belts is simple
if theres any question at all with it then change it
cost of a belt compared to cost of a top end rebuild when it breaks?
only makes sence to change it

oh and you say that its gauranteed for 100k miles,
no manufacturer actualy gaurantee's their belts, its just a recomended change interval,
thay always state that it should be inspected early and changed id required
so best to change it to be on the safe side
if it only had 45k when you bought it, then its unlikely to have ever been changed before