That will be tough but I think you can do it, but you will have to find a creative architect and a flexible builder.

You may have to source a lot of the items yourself and talk the builder into using your choices instead of his normal stuff. You may have trouble getting wood windows into the mix, and if you have to have masonry exterior that will be tough also.

Let everyone you are working with know what you are trying to achieve.

Start with city hall and find out what type of permits your project will need and what changes for remodel vs. new construction. In most cases, keeping any part of the old on a teardown will only hamper you in finding creative solutions for the new space. Talk to some builders about the general cost per square foot that their projects have come in at. You're going to be shocked at the prices. 250K won't be nearly enough for the project unless you choose a very simple box of a plan and go with builder grade finishes. Or, plan to create something that can have an addition at a later time so that you are doing the project in stages.

Unless there is really something special about this location or you have unusual requirements for your build, your money will go further to buy something existing that isn't a teardown.