Mortgage in Principal without knowing final property cost

I guess this may seem a bit odd, but we are first time buyers and have found a property which we like. We have made an offer which is lower than the current asking price and would like to obtain a mortgage in principal to show that we are serious and ready to proceed asap.

Without knowing the final price of the property is there any way of finding the best mortgage for ourselves and obtaining a MIP? Our offer is within 15k of the asking price if that helps :)