What are you looking for availability of? The Nodes, Applications, Components or Groups (actually called Containers in the database)?
For Nodes, use: AVG([OrionDB].dbo.ResponseTime.Availability)
For Applications, use: AVG([OrionDB].dbo.APM_ApplicationAvailability)
For Components, use: AVG([OrionDB].dbo.APM_ComponentAvailability)
For Groups(Containers), use: AVG([OrionDB].dbo.Containers_ContainerAvailability)
As for the business hours, use a nested AND section in the WHERE clause. Something like this:
((DATEPART(weekday, DateTime) >= 2) --Monday
AND (DATEPART(weekday, DateTime) <= 6) --Friday
AND (Convert(Char,DateTime,108) >= '08:00')
AND (Convert(Char,DateTime,108) <= '18:00'))