- best base harbors

I'd suggest Stavanger or perhaps Bergen.

- reliable and serious sailboat rental companies

Njaa Yachts http://www.njaa.no/ shouldn't do much wrong, and they're based
in the Stavanger area.
http://www.stavanger-seil.no/ (norwegian site with email-adresses and phone
numbers) has a couple of boats for rent.

- best time(s) of year

June or late august :-). "Everybody" has vacation in July and early august.
Biggest crowds in the ports on the southeast coast, but southwest / west
coast is also popular. More stable weather in late august than in june.

Good luck

Jorn Soderholm
Norway (Oslofjord / southeast coast).