I'm planing on setting up my own server but I have some things I would like to know about before I start and I hope you could help me with it.

First is about softwareRAID in linux, I planing on using RAID5 or RAID6.
Can I add more drive to the RAID to expand the disk space or do I have to rebuild the raid and erase all data to do this?

And then I'm pretty new to linux I would like to have remote (web?) GUI to administrate following programs is there something that enables that for me, and witch program is the best for the tasks?
VPN server
Samba server
FTP server
Mail Server
Firewall
SoftwareRAID

oh, btw planing on using Ubuntu or Debian if no other better are suggested.
And last is there something critical I should know about RAID or any of the server software?

For a server, hardware raid, *real* hardware raid not firmware or fakeraid is probably better. Better speed, possibility of hotswapping with a rackmount, lots of advantages.

You can usually grow software RAID without difficulty; try googling "grow raid" Of course if you have a dedicated hardware array, it will have its own limits and procedure - but it should be relatively easy as well.

Remote control from the list you have there: VPN

Debian or Ubuntu; either one is fine Ubuntu is designed to be easier for those who are pretty new to linux, but let's not get into a distro comparison fest. Either one is fine.

Last question: read a lot in advance