some questions about factions and nightfall

I don't have prophercies(actually I had it and played it for quite sometime but I no longer do).Now I'm planning buying nightfall or factions or maybe both.
some questions:
Let's say if I buy nightfall alone do I have access to all the professions? Or just the two new professions(hopefully not)? Or the 6 in prophecies+the two new? What professions do I get if I buy factions alone?
How many skills do i have access to? like if I buy nightfall alone my profession won't have the skills added in factions will it?Do I have access to all the prophercies' skills?
How large is the world in factions and nightfall? How long are the two games?(the main story line)

All Campaigns come with core skills & core classes. Every campaign has unique skills.
http://gw.gamewikis.org/wiki/Skills_by_Campaign shows what skills belong in what campaing & what core skills there is.
For how large & long every campaing is? Well that is relative but in the campaings other than Prophecies you become level 20 fairly fast.

your prophercies account is still there waiting for you to add a new key to it.

if you install Nightfall/Factions as a separate game you only have a core set of skills from prophercies and not all of them plus the new ones from each chapter

on linked accounts you can bring characters from one chapter to the other(s)

my opinion Nightfall beats Factions in every way for PVE

longer/more to do/heroes/etc

also be advised that if you don't have either of chapters 2 or 3, you wont have access to the new classes introduced in the respective campaign.

so, in your example, if you have prophecies, buy nightfall, but skip factions, you will not be able to create either an assassin or a ritualist.

you will have all the core classes.