With yahoo, you can always choose custom install, just choose the core application and you'll be fine. And with Skype, yeah, you can use it for IMs too.
Yes it does come with instructions and yes you will need a wireles receiver for every PC that is not wireless capable.
You can use WPA encryption which you can set using the routers admin panel. If you dont want your wireless network to be visible in the wireless list, you can just simply...
Yup thats what im using on my aunt's, no problem, never restarted for about 7 months now. That was v1 though, I think its on v3 now but should be fine.
Unfortunately vBulletin only accepts Paypal and Credit Cards.
The basic installation of vBulletin is about 20Mb in size, so a 100Mb hosting would be enough, unless you want to host attachments, avatars, signatures, files etc.