Hi Mark
One of the things you need to bear in mind is that although in Italy building regulations are covered nationally by the Testo Unico Delle Disposizioni Legislative e Regolamentari in materia Edilizia, there are regional variations so it would be unwise to make comparisons between southern Italy and Tuscany for example. From experience, Tuscany is far more rigid in its planning rules and it pays to have the proper, local advice from the start.
If the property needs renovation, a good Tecnico (Architect or Geometra) and builder is Key. However, in order to avoid any potential disputes in the future, make sure that everything is in writing and do not enter into informal agreements with builders or project design specialists. Don't make the same mistake that many foreign nationals do, who invariably end up being involved in lenghty and costly litigation.
The things to look out for when viewing buildings in need of renovation should include an inspection of the roof, walls, floors, celings and exposed beams and joinery. I would recommend that you get a perizia giurata and be specific about what you want in the report. Some surveys I have seen amount to nothing other than a simple description of the property. You need to know, among other things, what potential problems are lurking in the background, what will be required to remedy them and an indication of what it is likely to cost.
I actually prefer stone built than brick, however, exposed stone walls present their own challenging quirks and care should be taken to protect the walls from moisture and damp.
Most rural buildings that you see scattered around the Tuscan countryside do not have foundations. Morever, seismic regulations (For example, Norme Tecniche per le Costruzioni D.M. 14/09/2005 e Normativa Sismica: Ordinanze 3274/2003 e 3431/2005 refers) apply to all of Italy - again the Architect and structural engineer will be able to provide specific information pertaining to the region and location where you decide to purchase. Remember: Legislation in Italy changes frequently and there are always new laws: for example, a new law was introduced in February of this year concerning the technical norms for using steel in the construction of buildings in seismic zones.
As far as the B&B operation is concerned, again bear in mind that regulations vary regionally and it is better to speak to someone operating in Tuscany. I know of a number of people with B&B facilities there so if you need to so some research, let me know and I can give you some contacts. In the meantime, the Tuscan Regional Government law for B&B' etc is
LEGGE REGIONALE del 23 marzo 2000, n. 43 and note the changes to Article 55.