Well, instead of getting a sadde that you like without certainty that it fits your horse or not - why not get a Proffessional saddle maker/fitter come out and asess this horse you are leasing, so that you can be educated on what saddle will work for both you and your horse.
Just because you sat in it at a tack store, doesn't mean it is the saddle for you or for the horse you are leasing.
I think it is greatly irrisponsible and foolish to buy a saddle that you like, without knowing if it fits your horse or yourself first.