None - by the time you finish, you will have spent as much as if you had
simply purchased it.  Now to the technical answer - framework 3.0 or later.

.NET comes with most you will need.

I would add an O/R mapper like NHibernate or LLBLGen for persistance,
possible a DI framework like Spring.NET, log4net for logging, NUnit
for unit tests and maybe some more configuration management tools
(but that is outside my area of expertise).

Arme

Have a look at Enterprise Library (http://www.codeplex.com/entlib)
from Microsoft. It is a mature library covering basics like exception
handling, logging, validation or caching in a very complete, flexible
and modular way.
If your focus is on business objects have a look at Persistor.NET
(www.persistor.net) for persistence. It provides a very easy way to
store your objects.

Hans-Peter

Persistor.NET Team