Lots of general questions there, I'll try to touch a little on everything.
In general you should check out the Application Architecture Guide. It's a very good, general primer on architecting all sorts of .NET applications.
I don't think you are going to have any problem with Prisim. Take specific note of the MVVM Pattern. On the "performance" issue, I think what you are really talking about is "responsiveness". So a POS allplication needs to be very responsive to the user. Using WVVM, you can get very good responsiveness in your application.
Personally, I would stay away from this version of Entity Framework (although next version is looking good). I think LinqToSQL is fine (afterall, SO uses it :) Look at NHibernate for .NET if you want ORM.