[quote]
How often do you want players to post?[/quote]

As long as the flow of the roleplay can reasonably continue. Once a day or even once in a few days is fine, once every month isn't.

[quote]Can you give us a glimpse of this alternate history before we confirm interest?[/quote]

Oh, I have much more than a glimpse. Warning: TL;DR

General history of the world: [url]http://www.reddit.com/r/worldbuilding/comments/2xydhn/monthly_critic_thread_march_2015/cp6i42w[/url]
History and geography of the Viking world: [url]http://www.reddit.com/r/worldbuilding/comments/30695f/vikingligr_veldi_a_complete_history_and_geography[/url]
Short history of Greenland: [url]http://www.reddit.com/r/worldbuilding/comments/2wq4vu/tell_me_about_the_controversial_topics_you_have/cotlc9h [/url]
Periplus of the Eastern Sea: [url]http://www.reddit.com/r/worldbuilding/comments/2yholh/periplus_maris_erythraei_or_periplus_of_the[/url]
Traveller's Guide to Alexandria: [url]http://www.reddit.com/r/worldbuilding/comments/2x50hs/a_travellers_guide_to_alexandria_and_a_map_to_the [/url]
History and geography of the British Isles: [url]http://www.reddit.com/r/worldbuilding/comments/2wx6a8/a_map_of_britannia_in_893_ad_and_the_most [/url]
Map of the Mediterranean: [url]http://www.reddit.com/r/worldbuilding/comments/2wlamz/a_map_of_europa_in_ad_893/[/url]
And a general encyclopedia of the world (WIP): [url]http://www.terrachronica.com/worlds/715[/url]

[quote]Is this roleplay based around a small group of travelers, then, or individuals doing things around different parts of the world?

If it's based around a small group of travelers, is there a reason they'll want to stick together rather than separate due to conflicting cultures, worldviews and potentially religious friction?[/quote]

A small group of travelers. About the relationships among them, we'll figure them out as actual characters are submitted: players could be even in competition with each other.