A licenced general contractor can advise you what can be built, a local real estate professional can give you an idea of what the work will improve your property value by. Alot will depend on the quality of the work done, how it compares to properties of like type and the neighborhood value of your property currently and then after the work. You should also check with the building department/inspectional services to make sure whatever you do build is allowed & permitted. Some properties cannot have livable space allowed in the basement due to building codes & fire regulations. There's no real way around property tax increases, they base the increase on the building permits you take out for the work. If you don't get the proper permits you run the risk of having a property that may void your homeowners insurance, mortgage requirements & the town requiring you to remove all the work done.
Hope your project goes well,