I have a house in Toscana and want to install

It obviously really depends on whether there is existing serviceable plumbing, type of property, whether a boiler is needed, etc. You are better off just getting a quote from a local installer.

What will have a significant impact as well is the amount of assistenza muraria (builder's work) - cutting holes through stone walls, cutting chases in walls etc. 

An installer will calculate the cubic metrage to heat and tell you what you need.  AS for the radiators themselves, you can get aluminium elements (the cheapest) which you buy one by one and they are put together to make the right size for the room, or steel which is more expensive but lasts longer and comes in set sizes.   There are alot of variables.