To share files between the macbook and desktop you'll probably want to look into NFS (Network File System). Things like FTP are also possible for file transfer which is easier to set up then NFS but are more hassle if you use them (several times) daily. So look into NFS tutorials for Mac/Linux on google. Furthermore, if you happen to have MS Windows installed on your macbook and wish to access the desktop's file also, you'll need to look into samba. Samba is the UNIX way of dealing with Windows network folders. It is possible to use NFS on Windows but it's probably not recommended unless you have no choice for some reason.
wireless connection. *Use Desktop for a server for: Webpage, Forums, ftp
If by "webpage" you mean hosting a webpage on the desktop computer then you'll need a webserver. By far the most common is Apache but others exist such as lighttpd.
This brings me to the distro. Any of the distros you mentioned can do the above so it's not that important which one you choose. There are a million threads on this site with distro recommendations. If you'd like to look at one (which has a poll showing you which are the most popular) then look here
. All of the above programs (FTP servers, Samba, NFS servers, etc) can be installed through the distro's package management system which varies but is usually pretty easy to use these days.