Your post has a confusing question. It's not clear if your looking for a steam boiler for radiator or boiler for hot water. A hot water boiler will run about 2k. In kind steam boiler should be 5-7k.

If you increase your BTU output, the plumber/city may require that you install a liner in you chimney for CO exhaust. If you have a liner, it may require a bigger liner that gets more complicated to run in your house.