It seems that there is no more 2 users for free in the SaaS Odoo offer. Free users that already had an account will still benefit from it but new users have only a trial period.
See this thread : https://www.odoo.com/fr_FR/groups/community-59/community-13162724?mode=thread&date_begin=&date_end=
And the founder answer : https://www.odoo.com/fr_FR/groups/community-59/community-13171402?mode=&date_begin=&date_end=
These are the answers of various people of the community on odoo forum:
https://www.odoo.com/forum/help-1/question/openerp7-windows-or-linux-what-is-fastest-18716
GNU/Linux better than windows but if you don't know anything about Linux, it may not worth it. ^__^
A new page has been created for documentation.
https://www.odoo.com/documentation/8.0/
Concerning your pricing, it really depends on to much factors.
Maybe check the OCA: